I love this game, and have for a long time. Play on PS4 and placed in every competitive season since the first (mostly Gold yo). I love playing Mystery Heroes, too: keeps you in practice with all heroes, keeps you on your toes, and yet it can be a chill experience as a solo rando, where team composition literally can't be an argument because you're not picking it!
It's that "doesn't have to be balanced" Mystery aspect, along with the rich History of balance and character changes to this game since 2016, that leads me to think a "Mystery History" mode--where you're randomly are assigned *any past version* of a random character--would be a fun way to remember that history.
Running your baby D. Va from her own ult... hauling ass as Original Symmetra to paint the scattering kittens with 75 shield each before you set up the Car Wash.... scooping up Life Orbs as OG Reaper... Mercy's HYUUUUGE REZZ! All sounds like fun to me! Memories...
Do you suppose the engine can support it? Don't you think this wold be a fun mode? :D