...Kinda creepy-lookin' Bats :-- !
There weren't many DC characters that were a good fit for Jack Kirby. However I can see in my mind's eye that Jack would have been a good choice for Aquaman. If you imagine Steve Rogers' head on Aquaman's body it might give you an idea.
Martian Manhunter, the Atom or Doom Patrol would have been interesting too.
You can see Aquaman, Batman, and Martian Manhunter drawn by Kirby in The Jack Kirby Omnibus Vol. 2 Starring the Super Powers. (Quite a mouthful.)
You are correct about Aquaman, Dandy! I'm also partial to his Dr. Fate and Firestorm.
I'll see your Firestorm and I'll raise you Commander Steel The Indestructible Man :-) (bit unfair, I realise).
How about Tommy Tomorrow? I can imagine Kirby handling him either in his 50s style or his 70s style. I like the idea of the 70s action-oriented Kirby doing the Martian Manhunter. The 50s Kirby would probably have put him into interesting SF adventures (if the editor had let him, as he might have been assigned a writer).
Incidentally, Kirby's cover for OMAC #1 recalls his splash for the second "Flying Fool" story here.