I have a lot to say about a lot of different topics, so I have decided to break this down into multiple posts.

Removal and Homogenization
The different factions of WI have different strengths and weaknesses which is an important part of the identity of that faction. In this post I will argue against homogenization of removal cards when a faction gets a card that does not fit in its identity or strengths (magic players would call it ‘color bleed’).
Although development and support removal has been restricted to specific factions each faction has some way of dealing with units. Here are some of the different unit removals and bounces currently available to each faction.

Orc : Troll Vomit, Smash ‘Em! , Foot of Gork, Easy Pickins, Lobber Crew
Chaos : Plague Bomb, Branded by Khorne, Seductive Delusion, Sorcerer of Tzeentch, Warhounds, Cloying Quagmire
Dark Elf : Invoke Khaine’s Wrath, Toxic Hydra, Sacrifice to Khaine, Call the Blood, Black Horror, Har Ganeth, Seasoned Corsair, Offering to Hekarti
Skaven : Deathmaster Sniktch
Undead : Wight Lord, Blood Dragon Knight, Danse Macabre

I am primarily worried that the DE unit removal cards are overlapping with the themes of other factions. Although Black Horror is better than Foot of Gork, they are too similar since they both focus on cost restrictions (an Orc theme). Also, Lobber Crew is better than Sacrifice to Khaine, and both are too similar. Call the Blood destroys damaged units but this fits thematically better with HE or Chaos. I want the DE to get removal that fits better with their current themes such as hp loss, power loss and card discard.
But hp loss should not end up into simply a better version of burn which is why DE are better at dealing with dwarves. There also seems to be some overlap between Undead and DE in hp and power loss. I suggest that DE focus more on power and hp restrictions instead. Good examples of hp restricted removal and bounce are Deathmaster Sniktch and Har Ganeth. Some suggestions:

DE 0DD: Tactic. Destroy target unit without any power. (works with Morathi, Bloodcall Sorceress etc.)
DE 2DDD: Tactic. Destroy all units without any power.
DE 1DD: Tactic. Return target unit with 1 or fewer power to its owner’s hand.
DE 1DDD: Tactic. Play during your turn. Look at target opponent’s hand. Choose and discard a unit card from that hand.

Skaven 2: Tactic. Destroy target unit with fewer HP than the number of corrupted Skaven units you control.
Undead 3: Tactic. Target unit gets -3 HP until the end of turn.

Empire : Judgment of Verena, Chain Lightning, Manaan Take You! , Surrender, Called Back
Dwarf : Master Rune of Spite, Slayers of Karak Kadrin, Honor in Death
High Elf : Flames of the Phoenix, Judgment of Loec, Moon Dragon, Banish

Here chain lightning seems out of place because it destroys attacking units based upon their printed costs. Empire should instead focus on its specialties such as tactics and attachments that grant counterstrike (dealing damage to attacking units instead of outright destroying them like Dwarves), new versions of bounce and a cheaper version of manaan. Examples:

Empire 2EE: Tactic. Play during your turn. Destroy target unit in a zone with no developments.
Empire 1EE: Tactic. One of your target defending unit gains Counterstrike 2 until the end of turn.
Empire 1EEE OR 2EE: Attachment. Attached unit gains Counterstrike 2. You may play this card any time you could take an action.
Empire 2EEE: Attachment. Attached unit gains Counterstrike 3. You may play this card any time you could take an action.
Empire 1EE: Attachment. If attached unit would leave this zone, you may instead sacrifice this attachment and put attached unit on the bottom of its owner’s deck.

I think that Slayers of KK is too strong, it needs errata and dwarves need more balanced cards in its place (like Beleaguered Scout). HE should get more thematic ‘cancelling’ cards (like Asuryan’s Cleansing) and cards which have synergy with indirect damage. Some early HE heroes had a theme of returning units to their owner’s hand.

Dwarf ‘Explosive Trap’ 2DD: Tactic. Target player attacking your capital must sacrifice one of his attacking units.

HE 1HH: Tactic. Return target attacking unit to its owner’s hand at the end of this turn.
HE 2HH: Tactic. Cancel and discard a unit card just played.
HE 2HH: Tactic. Each opponent must sacrifice a damaged unit he controls.
HE 2H: Tactic. The unit in play with the highest number of damage on it must be sacrificed. You choose which unit in case of a tie.
HE 0HH: Tactic. Return target damaged unit to its owner’s hand.

Wood Elf : The Wild Hunt
Lizardmen : Born Predators, Loqtza, Skinks of Sotek, Spawn of Itzl

I suggest that one of the Order only neutral factions get a way to deal with Toughness (preferably through uncancellable damage). Here are some ideas for Lizardmen:

Lizardmen 3: Tactic. Move all damage from one target unit another target unit.
Lizardmen 1: Tactic. Target unit you control gains Savage 2 until the end of turn.
Lizardmen 3: Tactic. Play during your turn. Deal 1 uncancellable damage to each unit.

Right now support removal cards are (besides Burn it Down)
Order : Demolition, Crafting the Rune; Judgement of Verena, Manaan Take You, Rodrik’s Raiders;
Destruction : Pillage, Dis Stuff Burns Gud, Snotling Saboteurs, Smash ‘Em All, Grimgor Ironhide; Embers to Inferno, Raiding Camps; Withering Hex;

The most worrying card here is Rodrik’s Raiders – it encroaches on the Dwarves’ strength in dealing with Support cards easily. Even if Empire does get support removal, it should be limited removal like manaan take you. Rodrik’s Raiders could get errata to allow Empire to bounce support cards. We haven’t yet seen cards that bounce or gain control of support cards. HE, arguably the weakest faction, has no way of dealing with non-Attachment support cards and could be given the theme of gaining control of support cards. HE card Banish could also be given errata to target supports or units to make it better.

Rodrik’s Raiders: When this unit enters play, put target support card on top of its owner's deck.
Empire 1EE: Return target support card to its owner’s hand.
HE 4HHH: Attachment. Attach to a target non-unique, non- Attachment support card. Take control of attached support. Move that support card to your corresponding zone.
HE 2HH: Tactic. Take control of target non-Attachment support card and move it to your corresponding zone until the end of turn.

Right now there are only a few cards that interact with burning zones. Dwarves have Grombrindal’s Elite, Grombrindal and Durgnar while Chaos has Embers to Inferno. More cards which get stronger or weaker along the theme of burning zones would be great. I also hope that these cards will also make sudden about turns during games more likely. Here are a few suggestions, and when use the word 'fits' below I mean that it fits thematically:

None of opponent’s zones are burning : Fits with Orcs. Also, Good for HE when opponent spreads out indirect damage among zones.
No zones are burning : Fits with Empire. Whether the unit gets stronger or weaker this will work well with rush decks.
None of your zones are burning : Fits with DE.
A zone is burning : Dwarves seem to be focusing on this already. I suggest Durgnar be given errata from your section burning to any zone burning.
One of opponent’s zones is burning : Fits with Chaos. ‘Embers to Inferno’ uses this already.
One of your zones is burning : Fits with HE. This will work well with HE cards which deal indirect damage to all players.

Suggestions:
Orc 2O: 1/1 Battlefield. This unit gains 1 Power while attacking if none of the defending player’s zones are burning.
HE 2HH: Deal 2 indirect damage to target opponent. Deal 4 indirect damage instead if none of that player’s zones are burning.
Dwarf 3DD: 1/4 Lower the cost to play this unit by 1 for each burning zone you control.
Neutral Unit 3: 1/3 This unit gains 1 power if this zone is burning.
Neutral ‘Contested Ruins’ 1: This card gains 1 power if this zone is not burning.

Here are some ideas for a few new mechanics.

DE/Undead Unit that inspired the Poison mechanic below: (1DD) 0/1 When this Unit is opposed in combat target opposing unit gets -1 hit points.
Poison X: When this unit is opposed in combat any number of opposing units gets a total –X hit points until the end of this turn. Note: This mechanic is meant to deal -1 hit point in the same way damage is assigned. It would work particularly well on units with 0 or 1 power. This was inspired by the unit above.

Neutral Tactic that inspired the Raze mechanic below: You may cancel any combat damage you assign to your opponent’s capital this battlefield phase. Then, for each damage cancelled in this way you may destroy a development in that zone instead.

Raze/Ruin: When this unit deals damage to an opponent’s capital and survives combat you may cancel combat damage assigned to your opponent’s capital equal to this unit’s power. Then, for each damage cancelled in this way you may destroy a development in that zone instead.

Hatred/Infiltrate: Take 1 resource from an opponent’s pool and add it to your pool if this unit survives combat. Note: This was inspired by the Scout keyword.

Vanguard/Escort: When this unit is declared as a defender, during the assign damage step of battlefield phase, damage must be assigned to this unit before it can be assigned to other units you control. Note: This will complement defensive abilities (Counterstrike, Savage and especially Toughness) by giving those units a higher chance to survive combat.

Loot/Raid: When this unit deals damage to an opponent’s capital and survives combat, then until the end of that player’s next turn unit and/or support cards in the defending zone lose power icons equal to this unit’s power. Note: This was inspired by someone’s idea of some sort of besieging mechanic on this forum.

Ward X (for Tactics only): If this card is face down as a development you may pay X resources and turn it face up any time you could take an action, then treat this card as if you played it from your hand. Note: This mechanic was inspired by James Hata’s original idea for Ambush as discussed in this link: http://www.fantasyflightgames.com/edge_npm_sec.asp?eidm=23&esem=1. I thought that it would work well on tactics.

Incantation (for Tactics only): When you play this tactic from your hand, put it into play face down as a development in one of your zones instead of your discard pile when it leaves play.

The above two mechanics(ward and enchant) could be combined in a few tactic cards to create recyclable tactic cards. Their effects should not be too powerful and they should be more expensive than the usual version of these spells for example a 1 cost seduced by darkness tactic with Ward and Enchant. Note: I think the ward mechanic fits thematically well with elves especially High Elf spells.

Support card inspired by the Enchant mechanic: Whenever an Attachment Support card you control would enter the discard pile from play you may put it face down as a development in one of your zones. Also, Ward could work on a support like this: Turn one of your developments face up. If it is not a tactic sacrifice it immediately. If it is a tactic you may play it immediately (paying all costs) as if it were in your hand.

WH:I represents some interesting themes as mechanics such as corruption, slavery (gaining control of opponent’s units from the discard pile) and delusion (units dealing damage to units in the same zone). I wanted to find a new mechanic to represent subversion and sedition, especially for Chaos. One idea is the loss of the loyalty symbols for the purpose of increasing costs. These cards still retain their loyalty cost so they should not count as neutral cards. Here are some examples:
Insidious Lies: (Tactic) Support cards controlled by target opponent do not provide any loyalty symbols until the end of turn.
Whispers of Power: (Attachment) Attach to a target unit or support. Attached card loses its loyalty symbol.