Short answer, I think both cities could support baseball and I don't think minor league park expansion or ticket sales are big obstacles.
As far as the lease, I don't know, but there is always a lot of legal maneuvering when these things happen. I just can't see the Rays being in Tampa 20 more years. If things continue, I could see a potential contraction/new franchise scenario.
Contraction is off-the-table, they already tried that and it failed. And remember of course, you need TWO franchises, they aren't going to operate with an odd number. The MLBPA would NEVER go for it. Not to mention with media deals and leases, that's a lot of lawsuits. Moving is MUCH more likely. Btw, I think I know where you are going with the contract then start again thing... the teams would get an expansion draft? I don't see it.
The cities would also have to commit to new stadiums, that's probably a bigger obstacle than anything I mention. You saw how difficult it was in DC for a time.
BTW, Sacramento said they wanted in on the MLB business since they are likely losing the Kings, I doubt anything happens there.
FYI, here are the top ten cities, Brown names(from 2008)... Charlotte/Portland/Northern NJ are at the top of the list.
http://www.bizofbase...rials&Itemid=39