Vixen: She’s to powerful to be a street level hero, but her general aesthetic and personality doesn’t seem to fit with a traditional cosmic level hero. She’s what I would call a world level hero. She should probably be fighting global terrorist groups, reasonably powerful supervillains and maybe some low tier magic threats. She should also be a globe hopper a character not tied to one city or country. This allows her to play up both her international appeal and her knowledge of the world’s wildlife.
Martian Manhunter: He needs to go more cosmic and weird. His powers are weird and almost tailor made for a Grant Morrison (Animal Man, Doom Patrol era Morrison) style series. His powers are very psychedelic when you think about it. DC needs a cosmic team. And Martian Manhunter should be on it.
Powergirl: Certain characters owe their very existence to the multiverse. DC needs a team built around inter dimensional travel. What’s the point of having a multiverse if it only shows in “event” stories. Powergirl and a lot of other characters that have inter dimensional travel in their history should form a team. Plus they need to stop making Powergirl just a palette swap Supergirl.