I'd want a Captain Marvel/Captain Marvel (none of this "Shazam" nonsense) crossover, especially with Captain Marvel, Jr. Of course, it would have to be written by Peter David.
I'd also want a Captain Marvel (Marvel version)/Firestorm team-up. Captain Marvel would be from the time when he shared the Nega-bands with Rick Jones and could talk with him, but no one else could hear. Likewise, Firestorm should be from the Ronnie/Professor Stein time when Ronnie could talk to the Professor without anyone hearing. You'd have two heroes who could consult with others, but no one else could hear them. The two heroes could swap notes.
They'd also have to reconcile there being a Green Lantern Corps and a Nova Corps.
As for Arthur and Namor, Fantastic Four introduced another Atlantean city that stayed hidden for centuries. They'd have to do something similar, with both Arthur and Namor coming from different cities that comprised Atlantis and stayed separate through the years. Perhaps the cities were at war at the time they drowned.