Disclaimers:
-This idea needs a lot of fleshing out, and may or may not be balanceable
-I suspect it won't be to the taste of many Dominion players
-I see no practical way to make this work physically
Still here? The idea I had was a Event that costs $0 and gives +Buy, but it could also be a variant by automatically (or optionally) occurring after every turn.
"Trash 2 Actions you have in play. Gain a Hybrid based on their properties."
The Hybrid would be a brand new card that would permanently inherit some of the traits of its "parents" in a semi-random fashion. My initial idea is that for vanilla effects like +Card, it'd have an equal chance of either parent's value or the average thereof. So a Smithy/Village Hybrid for example would have either +1, +2, or +3 cards, and +0, +1, or +2 actions.
For more complex effects, each parent could have an equal chance of passing on all, none, or part (where possible) thereof. So in addition to potentially passing on a +coin, a Militia's child might have "Discard to 3," "Discard to 4," or neither. Cards like Governor or Steward might pass on only one option. Stuff like Witch can't really be separated out, so you'd either get the Cursing or not.
In all these cases, once the Hybrid is generated, it stays that way. You don't know what you're getting exactly when you make a Hybrid, but once you've made it, it doesn't reroll. You could make another Hybrid with the same cards later in the game and maybe that one would be different.
Price could be an average of the two, or maybe the more expensive plus half the less expensive. You could also try to price each effect or piece thereof, but I think this forum has determined that down that road lies madness. I can't offhand think of a way for you to buy a Hybrid, so it only matters for card interactions anyway.
So some examples:
Council Room & Necropolis Hybrid would have:
(33%) +4 Cards (from CR)
(33%) +2 Cards (average)
(33%) No +Card (from Necro)
AND
(67%) +1 Buy (from CR and average)
(33%) No +Buy (from Necro)
AND
(33%) No +Action (from CR)
(33%) +1 Action (average)
(33%) +2 Actions (from Necro)
AND
(50%) Opponent draws a card
(50%) Nothing
Grand Market + Steward
(67%) +1 Card (GM/Average)
AND
(67%) +1 Action
AND
(67%) +1 Buy
AND
(33%) +2$ (GM)
(33%) +1$ (Average)
AND
(50%) Can't be bought with Coppers in play
AND
(33%) Steward effect
(33% * 33%) +2 Cards
(33% * 33%) Trash 2
(33% * 33%) +2$
For a less random variant, you could narrow it down to only averages and parts where available. Under that version, our CR/Necro example above would then be a Lab with a +Buy that may or may not have the opponent draw. The GM/Steward would be a Market (if halves round up) with a random Steward option and a 50/50 shot of inheriting the Copper restriction.
Does this idea even make sense the way I've presented it? Is it something anybody would want to bother with thinking about?