I like to think that West Coast 4 will take place in Utah.
Bottom of the map cuts off right at Zion National Park. Top of the map might have a sliver of Idaho, probably Malad pass.
The main factions are the largely-destroyed new Canaanites in the south and some remnants of the Master's army in the north.
Minor factions are the descendants of various extended families who were prepared for the end (the Mormon church stresses the everliving fuck out of disaster preparedness, for those of you who don't know), but most of them are ghouls. Maybe West Coast 4 is the game where there's finally shit-tons of ghouls.
They'd probably have to tiptoe around the presence of Mormon temples, since the church isn't really big on being represented in media by anyone but themselves. Maybe they built a Vault with all that tithing money and turned the General Authorities into Robobrains. Or they're doing a Tranquility Lane thing. shiver
There's Salt Lake and its satellites to explore, as well as a Fallout version of the Lagoon amusement park, probably some sort of Captain Cosmos brand deal.
I've always wanted a new Fallout game to have a quest where you discover music and deliver it to have it played on the radio. The main problem with that is getting the rights to enough music to make that a viable quest. One way to get around that is by doing your own cover of the song. IIRC you own the rights to your own performances?
Well, American Fork high school has one of the best marching bands on the west coast, so besides the main Fallout-style radio station, they'd have a second station with a bunch of ghoulified band nerds broadcasting marching covers of various songs from previous games. This is the station you'd deliver sheet music to, and they'd transpose and play it.
The Utah Renaissance Faire is pretty big, and maybe I missed something from not playing Fallout 1, but AFAIK we have yet to have a re-enacter faction (besides the Legion, I guess?).
People get into it to different degrees, too, so one of the faction quests would be to decide who gets to run the faction: the playacting nerd who likes to j off over his knighthood, the really anal period-accurate guy, or the weirdly aggressive live steel girl.
Also more intelligent Deathclaws, please. Goris is m'babe.