Playable races:
Neds/Nords- I can't remember which it would be but it could begin when they had they're small empire in skyrim or when they sail from at oar with Ysgramor.
Dwemer- Dwarves would still be around and they could possible be a good race to play as. Lots of hate with the Chimer
Chimer- they're the ancestors of the dark elves and would be the jack of all trades in this game
Snow elves- they would be from skyrim and be good mages and a cross between woodelves and Altmer from later eras
Alyieds- the incarnation of the Altmer in this game, they would be the most prominent race as they control most of Cyrodil and Alinor
Imperials/ Alessians/ Slaves- they could be the major human race in this game working on Alyied constructs
Beast folk- they would probably be the same as later games but maybe more primative
Orsimer- I forgot the old name>.< but they would be a bunch of nomads led by Trinimac (Boethiahs quest could possibly involve locating Trinimac and giving him to Boethiah)
Main Quest:
The main quest could possibly the uprising of Alessia or an earlier failed uprising.
I'm not sure how this would fit in lorewise possibly being another of Alessia's Champion or even playing as Pelinal himself.
This is probably just a concept but I'd love to here your ideas and opinions so please feel free to reply^.^