C&C4 definitely was a missed opportunity. While this wasn't a bad game, it shouldn't have continued on with the Kane story. Using the traditional C&C styled gameplay to do Tiberian Twilight would have been much better. The acting isn't cheesey enough either. C&C3 was a much stronger effort.
The gameplay lends itself to the Generals theme better than the Tiberium universe IMHO... could have had a separate crawler for the different Generals which makes sense.
On the other hand, RA3 Uprising wasn't a bad expansion. A few mini campaigns, etc is a nice addition... though, I never did find RA3 to be as Epic as RA1 or RA2.
I'm really hoping that they make Generals 2 now... after trying Starcraft 2 as well, if they do a Generals 2 with updated (not radically changed) gameplay mechanics to bring it on par with that of Starcraft 2's, I think we may have a winner.