Legionnaires can visit when time travelling (I love comics) but having them be a permanent member is weird, because I feel like their story would always be about trying to get home.
Global Guardians would be good adds for diversity, but they're all barely knowns and a lot of them seem like *better known character's power set from another country*. It feels like tokenism and they don't fit in with the big guns.
However, the cartoons (both JLU and Brave and Bold) showed incorporating Vigilante could be a hoot, so he's the only one that gets my vote since the last time I chimed in.
For the most part, I prefer long time members of other teams to stay with those teams other than possibly as a liaison position.
However, the Earth-1 Vigilante does have some history with the team. He was there for their secret secret-origin. And, he had appeared with the JLA for a mission as well as teaming up with Superman in World's Finest.