Glad to see the short-lived [channelf] mailing list spawned one dedicated coder! I gave up on the F8 a long time ago.
Anyway, I think there's a lot of potential left in every system out there besides the ones that have been extensively developed for--I think in the next year or two we're going to see games start to hit the actual technical limits of the VCS, as in, we'll run out of new dirty tricks (at least on the hardware side without new bankswitching schemes). I would figure this will bring about another generation of homebrews, with really weird gameplay, or maybe even homebrew peripherals (AVox leading the charge, maybe we'll see new controllers, etc)... or maybe all of the innovation in the VCS space will move into software. There's some talk of this in the comments of one of Thomas' recent blog posts. This is what excites me the most, a new way of approaching coding the VCS.
That, or ColecoVision, will be the future of homebrews. Though I'm still pulling for Game Boy.