next TES

you ought to be able to make multiple characters on the same save
only one of them would be the nerevarine/hero of kvatch/dragonborn/whatever the hell else and that would be the only character that could finish the main quest on that save
other characters could still do other stuff and you'd be able to find them in game etc
like if you became archmage of the mage's guild on one save and visited the mage's guild on another character you'd see your archmage character there doing archmage stuff
also if you joined the mage's guild on that second character you'd have an alternate version of the quest line that doesn't end in that character replacing the first but you still get some cool stuff

i realize i'm basically describing a single player offline MMORPG but that's basically what TES is anyway