Your best option for 30-team Franchise control is definitely NHL 2K10 on the 360. Don't get the PS3 version; it has a bug that seems to systematically lock up Franchise mode after just a couple of seasons.
EDIT: you could also wait for
DJ Neo's mod for PC to come out and just get the Wii version. But you'd have to ask him if 30-team control is possible... I'm guessing it is but not sure.
Molifox also stated
here that the 360 version of 2K8 has 30-team control, but only if you download a patch. Again, avoid the PS3 version; it doesn't have this feature. Even 4-team control doesn't work properly on the PS3 version.
I doubt it. The game has to store every team in memory, even if it's a CPU team. Maybe the devs intended that feature for 4-player multiplayer when it was introduced, and never improved it since their focus was on the 360 after 2K5.