Donald has had "endless duration" cards as well, but they turned into
self-topdeckers like Treasury and Alchemist.
Treasury: [...] eventually settling on a card that you can have every turn if you want, which had been something I'd been meaning to get into the set but hadn't. Instead of sitting in play, it replaces itself (+1 Card +1 Action) and goes back on top of your deck each turn. The "but not if you bought a Victory card" clause keeps it more interesting and fairer. I had a version that you trashed when you bought a Victory card, but it was fine to let you keep it.
http://forum.dominionstrategy.com/index.php?topic=117.0Alchemist: [...] I tried a card that just increased your hand size for the rest of the game, but that would have required a playmat, and the set couldn't have any. This is the next best thing, and more interesting due to the Potion requirement. [...]
http://forum.dominionstrategy.com/index.php?topic=118.0So if it doesn't hurt the idea, you could do something like this:
Name: Unnamed
Type: ACTION - DURATION
Cost: $5
+1 Card
+1 Action
Choose one: +1 Action; +1 Card; +1 Buy; +1 Coin
_____________________________
When this is in play during your buy phase, you may pay $1. If you do, put this on top of your deck.
This would give you the option not to pay the extra $1 which means you don't get the benefit until you draw it again.
I think the options are too many. You should not have automatically everything in the kingdom (Village, Draw, Buy), this would lead to less interesting games because you can do anything you want. You should exclude at least one of the options. (I would cancel +1 Card because it is so strong.)
Edit: Rereading the Secret History of Alchemist, maybe there will actually be "endless durations" in the new Adventures expansion since there are going to be player mats...?