My 2 cents. Pro mode is pretty much fine as is, except that goko (or making fun, I guess?) really needs to implement automatch and a proper lobby system. There shouldn't be any kind of ban list or anything. In my mind "pro" means all-cards, full-random, no knowledge of the kingdom before hand. Obviously people having different set ownerships messes with this somewhat, but I don't really see any practical way to fix that. I don't think it's fair to exclude people who don't have all the cards from hosting pro (especially, what if they have all the sets, but not all the promos. Would that be enough?). Anyway, I don't see a practical solution to that issue. I do think that it makes sense that "pro" mode should be aimed at the interests of the competitive players though.
I think the real problem, as mentioned in the OP is that playing casual "it'd be hard to find an opponent because all the better players seem to just play pro." I think if casual were fixed up a little bit then perhaps people would be more willing to play it too. I probably would still only host pro-mode matches, but I'd be willing to join casual games if it were possible to see what criteria someone used when picking the game (including what sets were drawn from). This would require goko implementing a server-side functioning kingdom generator. LF points out that someone could just keep generating kingdoms until they got, say KC/Goons/Masq, which I guess is an issue, but I think if you were only allowed to put in your parameters and then you didn't see the kingdom until you hit "create game," then generating kingdoms until you got the one you want would be a sufficiently large hassle that it wouldn't be that much of a problem. Some dick would probably still do it, but, oh well, this is casual mode and I don't think it would happen very often.