In most rpg you would be able to side against the gray fox and the thieves guild instead of joining him if you wanted to, same with the dark brotherhood... in fact I thought about this when playing oblivion for the first time
I have never done much with the construction set (just adding objects to cells and npc + changing stats...a little of scripting too but not much) but I might try it if it hasn't been done
In my mod the legion would be like the other guilds, with ranks and stuff... the first quests would deal with minor problems and soon you'd start helping Hieronymus Lex with the thieves guild... eventually capturing the gray fox! Or maybe not... I'll think about something interesting xD You'll deal with the dark brotherhood too, finding the cheydinhall sanctuary would be cool... I play with OOO/FCOM so the quest would be balanced for them (I mean, you wouldn't be able to do the last quests until high levels) I'll have to look at the lore too
So yeah, I'd use vanilla places and npc... no new fancy items I guess
there're some problems though
1. My english is not very good
2. I don't have that much time
3. As I said I'm not very good with the construction set... yet
I have not started and I don't know when, if someone who's actually good at modding wanted to help it would be amazing!
Of course, if it's already done it would be even more amazing!