Would you want it to be literally every card ever, or would you want it to be more like M:EM where stuff has to go through sort of a voting procedure to get in?
It actually surprises me that you don't have a voting committee thing like that for YMTC already in some ways, and it might be worth considering.
I can also see why you'd want to keep things very open and free form, though, since card design takes so much collaboration and revision.
I really like the idea of being able to trawl through the card lists for stuff M:EM can use! You might want to autoinclude specific permission levels for sharing, like we do for characters and settings, just so there's no misunderstandings. But yeah, it's always been hard for me, at least, to pinpoint what I should try to pull from YMTC to bring into M:EM so a central database of sorts where I could see larger-scale projects, in particular, would be fantastic.
I would like most of the cards to get in or at least the ones that user's submit themselves. YmtC is extremely casual, like a really big creative posting game. We don't really have a lot of structure or resources for big sets and most are content with just posting the occasional idea since sets can be such work. We have regular nominations for Card of the Week and Card of the Month but other than that nothing of the sort that M:EM has; someone posts a card, if its a contest it gets graded/commented on and if someone really likes it they'll nominate it, but that's usually it and we move onto another. Thousand monkeys thousand typewriters deal. I think we're okay with that though. I am mostly thinking forward for the future of the site when we have more members and more people active and interested in such things.