I suppose it isn't reasonable to expect as great an improvement as there was going from 3 to 4, but I can't help fearing that this isn't entirely intended to be a major release the way 4 was.
The main change that's being discussed is that the tiles are going from square to hexagonal, which is a big enough change to alter current strategies, I'd imagine.
I don't care if it is incremental. I've owned every Civ game since the first one. I don't see that changing.
ETA: Also, it's been 5 years since IV was released, and 3 years since the last expansion. Just because we didn't hear about it until now doesn't mean they haven't been working on it for a while.