I fucking love these 90s colors.
There's some stuff I could draw on to fluff Earth-3's future eras like the Super Syndicate from One Million. I definitely want to throw in Waverider and an Armageddon 2001-esque little metaplot. But beyond that, I feel like time travelers from the future could undermine some of the roleplaying potential of Earth-3. If everything is shiny and lawful in the 30th century, would Underground or Syndicate grunts really feel the need to keep fighting?
While such issues can be handwaved in static fiction, I think it infringes a bit on player agency and emergent storytelling in a roleplaying game. Therefore I'm putting a bunch of characters I like (Booster Gold, O.M.A.C., the Legion, Zoom) into cold storage. It feels like I'm cutting out some potential fun but I think it's for the best.
On the other hand, I think that allowing time travelers from the past can add a lot to the story. A character from a pre-Syndicate era can be horrified or awed by the modern world while also providing a different perspective to present-day mutahumans.