I'm actually a really active Sim City 4 player, I've downloaded a few thousand lots and mods and been playing regularly since SC4's release. I'm into the game, I love it.
Its hard to tell where Maxis is going with this, obviously it isn't going to be Sim City Societies 2 (it was such a trainwreck, they'd be stupid to try something like that again) but at the same time not everything they've said is encouraging. Deeper levels of simulation are unlikely, more functional eye candy (huge airports and seaports built piecemeal, full implementation of the HSR and NAM mods) is probably going to be a mixed bag and they are not going to include anywhere near the current amount of lots available for SC4 on the fansite exchanges in their tilesets.
BUT
It will be moddable, which is a huge +. What really matters is how well the make the game to not be repulsive to serious players visually and allow us to incorporate the mods into the structure of the game. Transportation is where they failed miserably in SC4, the community made it tolerable, but we never got that full experience. All Maxis needs to do is give the foundation for a deep and realistic city sim, the community will gladly do the rest.