I believe there's a decent amount of prior art for games supporting cross-platform play across Steam and traditional consoles (PS4 / Xbone / Switch). Presumably

I believe there's a decent amount of prior art for games supporting cross-platform play across Steam and traditional consoles (PS4 / Xbone / Switch). Presumably you'd need to re-buy a game like Rocket League to get it to show up in your Epic library, but the Steam version could likely be patched to allow Epic players to play alongside Steam players?