Spiritual successors? So if I said Doom I could have Halo, CoD >3, Serious Sam, Quake, etc, etc, etc, etc?
Keep it reasonable, ey?
I'd go with Command and Conquer. I like Fallout 3/NV (spiritual successor to Borderlands? Huh? Huh? ;P), I like Mount and Blade, I like.. Quite a few games. King Arthur the Roleplaying Wargame, Total War.
Command and Conquer just manages to go from fun to weird and wonderful whilst still keeping the same feel. Generals was grand fun, especially with unlimited super weapons. Carving your name into a turtles base with particle uplink cannons was hilarious. Zero Hour made it easier, yet harder. Money was slightly more easily acquired for the Chinese, various techs, all that. But that just means armies were bigger, were hitting more often and it could be hell against a decent player.
Then we have Red Alert 3.. Transformers vs parachuting polar bears vs.. Allies. TBH I didn't play them much, parachuting polar bears and transformers were much more fun. And having Akira available for play is pretty cool.
Or there's Total Annihilation>Supreme Commander, combined with mods those games are grand.
I only recently worked out the answer to "what CD", what game may take a while. But C&C in the meantime.
EDIT:
Can't seem to spell tonight
Second Edit:
I lied, Europa Universalis III is greater than C&C. KA: TRW and TW are greater too. So toss up between EU III, King Arthur and Total War.