Maybe in a future dlc, we can enter an arena and load a different save file to fight that character as an npc. Or multiple save files at once, maybe 3 max and they team up against you.

I saw a post in another thread where this guy mods his other characters into his game. So one is in an Inn, in the wild, and in the city. On PC obviously. I'd like to face my characters, sure. Only if they have all their perks though.