How powerful would this card be? (This is mainly a thought experiment.)
Wild Thing
Cost 4 - Action
+1 Card, +1 Action
You may discard a card you have in play.
In theory, this would let you play a card as many times as Wild Things you play per turn.
How much difference would it make if the +1 card was after the discard?
1. Trash down to 3 Wild Things
2. Play Wild Things until everybody dies
Better:
1. Put +1$ token on Wild Thing
2. Trash down to 3 Wild Things
3. Get infinite money
After adding a "That is not a Wild Thing" clause:
1. Inherit Wild Thing.
2. Play Estates until everybody dies
Not saying there's no wording that avoids infinite turns, but that's something to look out for. Considering that you will usually have either no use for this or it will be absurdly good, i simply think it's not worth the issues it creates. Durations also jump to mind as something that's going to be awful to track (how often did I play this Hireling, again?), but in fact any card that gives +$ or +Action disappearing might be hard to remember.