Another idea:
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"Y-y X" (Call it Y X, Y-ing X, Y-ish X, or Y-ed X if it makes more sense. But you have to agree on the same name, so naming effects can work normally.)
Choose one: do X or do Y. If Y costs more than X at the setup, you may choose to do Y only once per turn.
Things other than the action part is the same as X.
X + Action, If X and Y cost the same, it costs one coin more; otherwise it costs whatever the more expensive card cost.
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Setup: Find two extra action Kingdom card piles costing 2-4. Add the first pile to the supply, cards in it called "Y-y X" where X is the original name of the cards in it, and Y is the original name of the cards in the second pile. If they're different in price, use the cheaper one as X. (The second pile is not in the supply)
Y cannot be self-referencing. (Currently only Rats and Cultists count as self-referencing. None cause big problem here, I think. But thematically a Ratty village is not a Rat. It just does't feel right.) X can be self-referencing. If it does, change all such references to "Y-y X". (So you still get Scavenging Rats, which gives you more Scavenging Rats if you wish.)
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Therefore, if "Y-y X" costs the same, it's like a Band of Misfits which can only choose X and Y as targets. But Y is not in the supply, so it is not strictly worse.
Examples:
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Chapel Moat (which is a little stronger than, say, Moated Chapel. But both are OK, I think.)
Choose one: draw 2 cards; or trash up to 4 cards.
When another player plays an Attack card, you may reveal this from your hand. If you do, you are unaffected by that attack.
Reaction, $3
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Smithing Bureaucrat
Choose one:
draw 3 cards; or gain a silver card; put it on top of your deck. Each other player reveals a Victory card from his hand and puts it on his deck (or reveals a hand with no Victory cards).
Action - Attack, $5
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Scavenging Rats
Choose one: + 2 Coins, You may put your deck into your discard pile. Look through your discard pile and put one card from it on top of your deck; +1 Card, +1 Action, Gain a Scavenging Rats. Trash a card from your hand other than a Scavenging Rats (or reveal a hand of all Scavenging Rats)
Action, $5
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Farming-villagey Worker's Village
Choose one: +1 Card, +2 Actions, +1 Buy; or +2 Actions; Reveal cards from the top of your deck until you reveal an Action or Treasure card. Put that card into your hand and discard the other cards.
Action, $5
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Otherwise, you get a card which costs as much as Y, but the Y ability can be used only once per turn. Other than that, you can always use it for the X ability.
Examples:
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Smithing Village
Choose one: +3 Card; or +1 Card and +2 Action. You may choose the first option only once per turn.
Action - $4
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Militia Great Hall (Which I think is a bit too strong. It's essentially Militias that don't hurt if they collide)
Choose one: +2 Coin, Each other player discards down to 3 cards in his hand; or +1 Card and +1 Action. You may choose the first option only once per turn.
1 VP
Victory - Action - Attack - $4
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Feasty Duchess (Awesome Duchy chains! Admittedly this is one of the more broken combinations I can think of. But doesn't it look so cool?)
Choose one: Trash this card. Gain a card costing up to 5; or +2, and Each player (including you) looks at the top card of his deck, and discards it or puts it back. You may choose the first option only once per turn.
In games using this, when you gain a Duchy, you may gain a Duchess..
Action, $4
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Well, I admit, this is actually two different designs packed together. The reason that, you have two designs, one wants two cards of the same cost, the other wants two cards of different costs. By merging them together, you just find two action cards at the 2-4 price range, so it's actually simpler.