Not much of a fan Generals and Zero Hour here. I've been a huge fan of C&C since the C&C1 demo back in my 486 days. C&C, RA, Tiberian Sun, RA2, plus expansions, were all awesome. I played a lot of RA2 online (was in the top 75 players at one point on WW Online), but I like all of them almost equally.
So I just installed The 1st Decade on my laptop, and though it would never run Generals anyway, I don't want it anyway. Classic ones are awesome though. Not sure on Renegades... played the beta or demo long ago and it was decent, but that's not a strategy anyway.
Outside of the ones you already have, I'd have to say try out Act of War. There's probably a demo for it or its expansion. One of the best modern RTS games IMO... well for a contemporary setting anyway. It didn't receive much mention, but got good reviews, and definitely worth a try if you're a RTS fan.