Thanks for your comments!
A Shopaholic would sometimes be very strong, with the right kind of engine and no other source of Buy. It would take along time to get to the point where you could productively use 4 Buys if there were no other sources, so the benefit could probably be relaxed to +2 Buys without it being too strong. If there were other sources of Buys then Shopaholic would have to have serious other advantages to make it attractive.
I guess you're right about +2 Buys being enough. And I hadn't thought about it easily becoming uninteresting if there is another source of +Buys. Not sure what to do about that.
A general problem with the alternate backs if you want to make physical cards: once you can identify cards from both sides it becomes more difficult to shuffle fairly.
They'd all be like Stash in that you may place them where you want when shuffling. That's not so fun with Easy Come, Easy Go, since you'd probably put them together (at the bottom of your deck).
For Colliding Worlds wording you could try "Reveal your hand. If you did not reveal a Colliding Worlds...".
Yes, that makes it easier that you are allowed to play it. It's just that the good stuff doesn't happen. If it has the Stash propery you wouldn't have to reveal your hand since everyone can see if you have it, and it could be just "Action: If you don't have a CW in your hand <good stuff happens>."
Then I don't really like it that if you play Village from Village+CW it doesn't get very much worse if you draw a CW. Then you'd just play it once without effect and then play it with effect.
So now I think it should have a negative effect when it doesn't have a positive effect. Probably the Stash idea isn't very good here, and it's better with the reveal mechanism, so
Action: Reveal your hand. If you have a CW bad stuff happens. Otherwise good stuff happens.
You point out that Discard Me has a built in Hag-on-Hag: is this a good thing?
I think it could be fun in a frustrating way. Ah, now I've drawn the good card. Hope I get to play it this time! Arrrghh, no, I had to discard it
again! (Good drawing makes it much better so you increase the chance of getting it in your hand during your turn.)
I'd also stay away from mandatory discards (although that was only an example).
Right. It could be any optional discard, since those with Discard Me in their hands would be forced to discard it then anyway. Like "Your opponents must discard one card or gain a Copper".