There’s a few Marvel characters that fill roles that DC doesn’t really have.

DC doesn’t have a big name patriotic hero so, as others have already said, Cap would be a good choice. I would definitely have him as the leader of the JSA, but with a twist. I’d move the team to Earth 2 again. Captain America is the moral center of the Marvel universe. If he comes to modern times that would be overshadowed by Superman, who is the heart of the DCU. Putting him and the JSA on Earth 2 would make him and the team the focal point of that earth. Having them meet for a Crisis every so often would make things that much more fun.

DC doesn’t really have a Hulk. Sure they have the knockoff character Damage, but I don’t think that character has any longevity or ability to change the DCU like the Hulk would. The Hulk is a wild card and putting him in on Earth 0 would be like dropping a (gamma) bomb right in the middle of the DCU. It would definitely make things a lot more chaotic. I think DC had a shot at making Grundy their Hulk-like character a few years ago with his great appearances in JLU and the Arkham games. Sadly they didn’t capitalize.

DC has often tried to find their own Wolverine. Hawkman has been pushed in that role a few times. Lobo, Timber Wolf, Pantha?, etc. It would be cool to see how Logan would fit into the DCU. Would like to see him and Slade have along history starting out in their military days. Made they came from the same super soldier/Weapon X program.