So, this proposal would lock me out from playing with something like 90% of the playerbase.
I'm not sure what you mean, but to be clear, matching wouldn't change at all; you just wouldn't have Possession show up when matched against someone who ban-listed it.
Sidenote: I like Chess, but i dont like the knight because it moves so akwardly.
You buy Scrabble. Turns out you don't like it. So you don't play it. Hooray, the system works.
You buy Alchemy. You don't like Possession. You don't have to play with it. You aren't all "I rue the day I bought Alchemy because now I have to play with Possession sometimes." You just don't play with that card you don't like.
You're playing online. Oops here's Possession. Ha ha, take that, customer.
It makes no sense to make people play with cards they don't like. Yes other people will then see those cards less often... exactly like in real life, where Joe speaks up and says "oh no I don't want to play with Possession, man, let's replace that one" and no-one says "suck it up Joe, besides we hate you."
isotropic had veto mode, where you would deal up 12 cards and then each player would veto one. I think that was bad, and that people came around there, did not end up liking it. Every game it would be, well there's one attack, I guess I ban that. The overall effect was like banning lots of cards; they could only show up if paired with worse cards. This isn't that. With veto mode maybe you'd be vetoing Militia for this game; you didn't put it on your 5-card ban list though.
Again I think the only concern with a 5-card ban list is gaming the system, and at 5 cards it's not a concern. The cards you don't get to see because so many people hate them, man, maybe I shouldn't have made those cards.