The Blue Beetle, sometime before he returns home after the end of World War II.
Damn Durlans!
Was Shrinking Violet/Yera swap the earliest high profile long term character swap/impersonation storyline in LEGION?
Protected by the Comics Code Authority
YES Capes. YES Masks. YES Secret Identities.
Wonder Woman in King's run. She's clearly been replaced by an A.I
I'd say Power Girl, for sure. This new take is...not great. Yeah, it might muddy the continuity even more, but arbitrarily chucking her entire secret identity for some rando new one is just flat out stupid.
Along the same lines, Fire in the most recent Fire & Ice mini.
Maybe the intelligent Man-Bat that was in JLD and is now a villain in Batman & Robin? Personal preference, of course, but I prefer Man-Bat to be the more tortured animalistic monster type, not the conniving genius villain type.
Future Huntress in JSA...in fact, I honestly wouldn't mind if most/all of the "lost characters" Johns introduced in the newer Flashpoint mini were actually part of scheme by a villain.
It would also be cool to see some things they attempted to do and then abandoned, just to explain out-of-character behavior in the past. Like how Booster acted in King's Batman run...maybe time travel really negatively impacts the aliens behind it so they abandon those plans because their Booster went totally off the rails.
Master Builder could be an interesting take because he doesn't need a ring or a battery to access his powers, if his identity is assumed after that happens. That would make it much easier to assume his identity vis a vis any other member of the GLC.
Max Lord so he can go annoy Blue Beetle or some other C-lister, just get him out and keep him out from WW stuff.
Batman. Shortly before he went all Batgod. His polymathic inventive genius is actually a steady supply of alien tech. His penchant for legions of sidekicks are (unknowingly) his cadre of planned human Janissary legion. Batman, Inc. was a scheme to stealth enlist numerous corporate interests to his schemes. His continued interest in the JL (and Supes & WW in particular) is to provide him the access necessary to neutralize the most likely threats to his masters' schemes (he tends to drop out when the heaviest weight members are absent). The Batclan has to have the aid of Superman and the GLC to expunge the imposter.
Mark Shaw. He was so great in the Manhunter series Ostrander wrote back in the day and whatever Bendis turned him into with all the Leviathan nonsense...no. No thank you.
Get him back to his old tracking down villains days with the kickass costume. Hell, pair him with Kate Spencer as mismatched partners and call the book Manhunters.
I wonder what the reaction would be if this was done with someone like Cyborg.