Hey there, I've been reading these forums for a while, but this is my first post, and its gonna be a lengthy one.

Me and my group has been playing Dark Heresy of and on for like 8 months, and over that period, we've tried Heresy, Deathwatch, RT and Black Crusade, but we've never had a really ongoing game that we were fully devoted too.

I decided to do a custom guardsman campaign, using new talents and mechanics.

Its a bit like a mini Deathwatch. Each player chooses the guard regiment they are from, and the bonuses that they would get for that.

The Story is that they are all left overs from battles that were deemed too wounded when their regiment moved on, and they were recruited into a special Inquisition/Imperial Guard joint unit. Their missions will be specialist things, like retrieval and assasination missions.
Missions are intended to be hard, with death likely, and fate points are set to 1 for starting characters. That way a character who scrabbles his way up to an officer rank is an achievement.

The way I'm planning the mechanics of all Guardsmen is, while only the guardsman career will be an option, at level 1 (May change to higher levels) the players can choose different career packages to vary themselves.
So far I have:
Commissar (Sort of a leadership char, Arbitres like. Not totally sure where to go with this character, as he wields a lot of power, compared to the others)
Medic ( Obviously the healer. Gets less weapon talents and is a bit weaker, but will have lots of healing bonus's),
Vehicle Operator (Like the medic in weaker at combat, but has lots of skills to make war machines even more deadly)
Scout (Sniping and Stealth. Pretty much assassin in guard form.)
Cleric (Fellowship, and inspirational. Has access to a few Faith Talents)

These classes will have access to just one tree of the guardsmen advances, and some of the talents will be removed, to balance people who play straight guards. Playing Straight guard will allow you to choose from all three trees, but at a slight exp increase for the other two. Also going to add talents to let them specialize better in heavy weapons.

I plan on stealing the reputation system from deathwatch, as well as hordes, and there will be a Rank system that is not tied to level. So a level 6 could still be a trooper, wheras a level 4 who has gone above and beyond in the service to the emperor could be a seargent.
Ranks will also be a career package, such as seargents getting leadership talents, or veterans squad getting access to melta weapons and training.

Vehicles will be operating mostly on the Rogue Trader/Deathwatch system

New Classes for Tech Use and Common Lores. So you cant use tech use to do anything from fix a bolter to hacking a computer
I'm trying to come up with a bunch of new talents for the side classes.

Some of the examples I have created for Medics and Operators

Operator

Tank commander: Full actions involving the command or operation of a tank become half actions. Cannot push the vehicle beyond its normal limits.

Vehicular Gunner: BS is at +10 when using a vehicles weapons.

Sentinel Requisition: You may start non covert missions with a sentinel equipped with a Heavy Auto gun, or Loading Claws

Bucket of bolts: You know how to keep a vehicle running past its due. Vehicles driven by this character gain 2 hull points. If a vehicle is rendered useless, as long as the total damage was less then 3 over the threshold, it can be repaired. Can be taken multiple times. (Sorta like vehicle sound constitution)

Tech Use: War Machines

Medic

Battlefield Medic: Suffer no penalties for using medicae during a fire fight

Master Surgeon: You heal an additional wound when you succeed on a medicae test, up to the total wounds from that injury.

Drug Bag: May start a mission with 4 doses of Legal drugs.

Hitting the right vein: Drugs administered by the character last a round longer.

Some of these talents would be available at level 1, and others would require higher levels and rank.

What do you think so far?

I'll post more when I get it written down.

Any ideas for talents, or feedback would be awesome.

Also, I wont have access to any of the books for two weeks, so forgive me if I make some errors.

How do you handle the Disruption?

What do you mean by Disruptions?

Sounds Great, my only question is will the bonuses to being straight guard be beneficial enough so players will play straight guard??

also what are they fighting?? I mean sure the ability to choose will be great but they may be made less effective by what they fight.. obviously it sucks to be the vehicle guy if you are fighting necrons...even with thier new rules

I hope straight guard will work. I'm sort of picturing them as the heavies, and adding talents for that accordingly, so they should be able to keep up.

For the first few missions they will be fighting an assortment of Xenoes, from Orks, Tau, and Necrons, but after a few preliminary missions, the main story will be a war against traitor guard and eventually chaos, hopefully culminating in the 13th black crusade.
I dont think the Vehicle class will have too many problems, as I intend to have every mission have a few vehicles laying around, and the Operator will also double as the engineer, since there is no tech priest, allowing him to repair and build things. That's also why I added the talent to let him start with a sentinel at later levels, so that they always have a vehicle near by.

Though it doesn't seem like this will be a problem, as I only have 4 players, and none of them chose Operator, ha ha.
Ah well, someone will die eventually and need to reroll.

Just the Operator and Medic careers are pretty iconic for a game called Amethyst, based on D&D rules that sounds somewhat similar to what you're proposing. The game features Disruption rules quite heavilly where magic and technology collide (to the detriment of technology, magic gets along just fine).