Back to the OP's idea, I'm in favor. It would be nice for the people who wrote the bizarre 'choose one' cards to explain how the options fit together, because a lot of them are inscrutable. (If it keeps people from submitting a random assortment of bonuses disguised as an idea, so much the better.) If a card is interesting but broken, it would be nice to frame the discussion in terms of the degree of the tweaks we can make without changing it to a different card entirely. This was the problem with Hinterland from the second contest, which had the silliest implementation of a cool idea. More text to wade through is a dangerous idea, but I think if it limited to 140 characters, and only allowed for cards that don't already require rule clarifications, it'll be bearable.
That being said, I disagree about Tilller: I think the price was reasonable, but just writing an interaction on the card doesn't automatically make it interesting. The worst card your opponent can pick is the most expensive one Remodel could've given you, so you lose all the flexibility but retain plenty of the strength, especially since it expands $5 cards. It's terrible at getting rid of copper, yeah, but so is Remodel.
You know what card your opponent will name in every conceivable situation, so you're only going to play it when he doesn't have a choice: you might as well write "Trash a card costing more than this. If you do, gain a Province." It'll get the same amount of play as Tiller (provided there's no $7), and it's still viable at $4, but I don't find it interesting.