[A new Red Alert]
[Make the next C&C a new Red Alert. The new faction is going to be the "Grey Invasion", a martian alien race. Have them invade earth and force the Allies and Soviets to co-operate, which, of course, won't work so well. Maybe the Rising Sun joins the fun, too. The Invasion could have all kinds of whacky units: shrink rays, growth rays, flying saucers, terrain modifiers, a tech replicator (copies enemy tech, but modifies it), land krakens (maybe they borrowed them from Yuri?), etc. They could have lines like "This is worse than Dagobah!," "Nerf-herders!," or "Resistance is futile," referencing Sci-Fi movies and shows. Also bring back special units that you earn by conquering an enemy tech lab.]
[I think the Red Alert series is still fresh, while Tiberium has, let's face it, kinda been done to death. The same is true for Generals, but Red Alert was always a favorite because of the humor (well, since RA2, I guess). You could do so much funny stuff with the aliens and the would fit the alternative 50s/60s feel of RA perfectly.]
[If you went with aliens, people could accuse you of copying from your own work (the Scrin from the Tiberium series). And maybe you'd fall from favor with the Generals crowd (but, let's be honest, you can't please everyone). Also, I don't know if it's a good or a bad thing to bring back the Rising Sun; 4 factions could be considered exessive. Not by me, though.]
This is what a "grey" look like (a classic martian "conspiracy" alien).