So in terms of competitive meta, the best decks seem to be focused on rapid aember generation and aember control, and decks that love fighting or building a board fall behind. So I came up with the Clash format to let people play with their fight loving decks, and maybe even with multiple people at once. The core mechanics are still there, but the game is otherwise played backgrounds.

Each player starts out with three keys and six aember. The goal of the game is to unforge all of your opponent's keys. If your opponent has no creatures, your creatures can attack your opponent directly, causing them to lose aember equal to the creature's power (minimum zero). Any creatures that attack an archon are immediately destroyed after aember is lost. At the beginning of your turn, if you have no aember, you must unforge a key and gain six aember. If you have no aember and no keys, then you lose. Reaping and aember bonuses effectively heal you, and stealing is a sort of life drain, and captured aember can cripple an opponent as well.

In theory, it seems like this format can work surprisingly well, and there are a couple of things I need extra opinions on or to test.
-The first is key cost cards. Key Cheats such as Key Charge can effectively earn you an Extra Life within a game (max 3), but then what about Grabber Jammer or Murmook or Lash? In this format they only exist to make Key Cheats harder. I've thought of the idea of being able to forge a key if you have aember to spare at the beginning of your turn to stave off your defeat, but without testing it, I feel like this could make Clash games run very long.

-The Second is Ability Damage. There are a few ways to go about it. Either ability damage can only effect creatures, or it can affect an archon if there are no enemy creatures, or it can target an archon whenever. I'd need to test to be sure, but I'm leaning towards one of the first two, mainly the first.

If anyone wants to try and let me know how it goes for them or if you have questions/opinions please let me know! This seems like a really fun format and I want to try and make it work, and possibly even popularize it.

When coming up with new ideas, it is best to keep it simple. Change the rules before changing how 350+ cards work individually.

Start with one rule change at a time and then test. Depending on what decks you have available, you are going to find that just changing one thing could break a card/strategy wide open.

Games tend to be more fun for all when you are able to do something versus having things taken away from you. This is why creatures always have the ability to reap, so when the more creature heavy decks play against those without things to fight, they are still able to generate aember.

Now as to your individual idea questions:

Key Cost Cards: You could just leave the rules about forging keys at the start of your turn alone. Your concern was about this making the game go longer, but that is what testing is going to show. Instead of damage reducing aember, how about a change to the number of keys required to win and introduce a "KeyTheft" house rule? Something like:

• "Once per turn, each player may exhaust and stun friendly creatures of the current house with a combined power of 10 or more to steal a forged key from an opponent."

This will let those battle-focused decks have something to do against those that generate all their aember by playing cards. They forge the cards and then you steal them. Adding the stun will make sure that you also have to go through some hoops and can't just play two Brobnar creatures and lock them out of the game. Feel free to increase the total power required after testing.

Ability Damage: If you choose to keep the "Damage = Aember Loss" route, I would suggest leaving ability damage to creatures otherwise you are going to introduce plenty of corner cases that you may not want to think about when they happen ( "Is my opponent on a Flank? There are no creatures to the left or right, but if they have two creatures, are they now in the middle of the battle line or does normal creature placement rules take place?" ). Ancient Bear has Assault, which deals damage only to creatures, but it can still deal its five power to the opponent [side note: Bear Flute might be OP if the creatures die after dealing damage] .

Were you also thinking things like Punch, Seeker Needle, and Cannon were able to go to your opponent's face/aember? Because those would just become extra key cost cards that would need to be addressed.

You could also introduce something much simpler like a house rule on creatures being able to capture aember by "fighting" instead of reaping, but you would need to figure out what a fair number is. At what point is it more beneficial for you to reap and try to end the game versus just capturing aember all the time and making the game last forever? Is it an ability you want to have open to every creature or a player ability?

Also, don't worry about if people aren't open to try your idea. Not everyone is going to like everything and that is fine for all involved, they don't need to play that way if they don't want to.

If you don't have many people to test with, or don't have time for full games, just flip over the top 12 cards (6 in play and 6 in your hand) and make a board state to see how the next couple turns would play out. If the games don't feel any different from a normal game, change one thing and try it again 10-20 times, and repeat. The key is only change one thing at a time to get best results, otherwise you are not going to understand what factor is contributing to your desired results.

I like game variants & am interested in trying out a KF multiplayer sometime.

Maybe instead of keys, think of them as "shields" protecting your opponent (who has 1 HP). Your creatures have to bring down the shields to get to the opponent.

Once you think of it that way, the ruling on cards will probably fall into place. You can then think of aember as either parts to assemble a shield (in a fantasy sense) or power to bring one online (in a scifi sense). Normally you'd need time to assemble a shield (hence the "wait until start, once per turn" restriction) but things like key charge are now emergency actions to bring shields online. Things like grabber jammer are increasing the effort your opponent has to spend to restore their shields. etc

It would also explain why creatures blow up after hitting at the opponent.

I'd run a test without the extra aember (just start with 3 keys that have 6 energy each, when they hit 0 energy, the shield "drops") first and see how that goes.