They've said they're mostly trying to keep the two games somewhat separate (like portions of the city exclusive to one or the other) so people can easily buy CoV separately or not buy it at all if they don't want to spend money on the expansion, but there are overlapping city areas where villians and heroes will be able to fight each other that you get when you buy both.

And who knows what the dev's have planned in the future. They've done some pretty cool things so far, like alien invasions, halloween special events, and the city freezing over with giant snowmen.

Unlike a lot of the other MMO's out there, City of Heroes gives out free expansions on a pretty regular basis. Well worth the monthy fee in my opinion.