Okay, look. The point is that it's not a Throne Room. One of the main features of this card is the delayed effect that you can get. Sorry, OneArmedMan, but that's a completely different card.
Another thing about this card is that if you have a cantrip in hand or something, it's not a dead card if you draw into it, unlike Throne Room. This card is sometimes better than Throne Room, but it's often worse - but it can affect Treasures too. I don't want to let people play Treasures in their Action phase or anything.
Also since it's a Treasure it probably needs a Treasure worth. So how about $0?
Memento
Action - Treasure
$4
Choose a card you have in play other than this. If you could normally play that card this phase, treat this card as if it were that card until this leaves play.
Worth $0
This means that if you choose an Action card during your Buy phase, Memento is just a Memento, and so you can activate Horn of Plenty with it - but that's about it. Not sure about the wording on the second sentence - it seems a bit wonky...