That's 3 factions. Plus a 4th because you know people will want to be Argonian raiders and form their own inept versions of the Vietnamese guerrilla fighters.
Hah, anyone stupid enough to try and play an Argonian after they see Aryon Llethri on the battlefield is an idiot.
Anyways, most of this will take place on Vvardenfell and the coast of the Inner Sea on the mainland, nowhere near the south border. Also, the Argonians haven't attacked yet, and probably wont for a good while. That's for an entirely different RP, which I also have a ton of ideas for.
And IB, I say we leave the Telvanni out completely. They wouldn't want to act as a mediating force anyways. They don't give a rats ass what happens to the Redoran or Hlaalu. Hell, they don't even care about themselves. Half the time they're too busy killing each other for power or hiding in their towers doing experiments.
This is what I have in mind. Post-CI the Redoran, Indoril, and anyone affiliated with the Temple, declare their independence from the Empire. Everyone has been hit hard by the OB crisis and very few of the Imperial legion remain. Hlaalu, in an attempt to gain much needed Imperial support to get them back on their feet (Major southern cities in Vvardenfell will have been destroyed and a lot of farmland burned leaving them between a rock and hard place) offer to aid the Empire in regaining control of the lands that seceded. The remaining Redoran leadership (Aryon Llethri, and Tidus's character, forgot his name) decide to lead a mass exodus of the house's remnants to the mainland after the destruction of Ald-Ruhn and the deaths of the entire Council. When the Hlaalu and Legion send orders to surrender many of the Redoran nobles that are still alive push the House to go to war and wipe out the outlanders and the traitorous Hlaalu. This puts a lot of political tension on the Redoran. The Hlaalu and Empire begin attacking the caravans of the Redoran as they head northwest to Khuul where a small fleet of transports from Blacklight are going to begin ferrying the survivors to the mainland. This prompts the Redoran into retaliating.
That's what I've come up with off of the top of my head. Things can be changed if need be, this is a very very rough idea and very flawed, but it's better to have something to work with than a blank slate.