[@Chiro] History of Legacies is going to be one we all figure out together, but the ones that definitely will exist, regardless of anyone playing the characters:

Justice Society of America (Pretty much all of the WWII era Superheroes, Captain America, The original Human Torch (The robot one) Wildcat, Alan Scott, Red Tornado (Ma Hunkle) and the like)
Xavier Institute for The Gifted (The X-Men, The Brotherhood of Mutants, Charles Xavier and Magneto.)
Superman (Yes, the big blue angel is going to be in this, why? Because he's my favourite superhero ever, that's why)
The 4th Wall-Society (Deadpool, Ambush Bug, Bat-Mite, The Mask, Etc. why are they here? Because they won't let me keep them out.)
S.H.I.E.L.D. and CADMUS (Two competing orginizations of the US government, trying to keep Supers under control. S.H.I.E.L.D. are doing it openly, whereas CADMUS are very much a dirty secret that doesn't officially exist.)