I also want to know more about the general philosophy behind introducing new lore with a Fallout mod, what should be avoided when doing so and what I should do to make sure things introduced in my own mod project fit in with the Fallout setting.
My cents, its something I've discussed with The Dutch Ghost as well.
1. Don't be too unoriginal. One criticism I've seen, both at most big mods in general and Bethesda, is that they seem too afraid to innovate. They're basing Fallout on Fallout, but Fallout wasn't based on Fallout, it was based on other things. FO3's main story suffers from being a big mish-mash of FO1 (main quest around Water, Super Mutants, Vault as central), FO2 (Enclave as main villains, GECK) and even a bit of FOT (Super Mutants vs Brotherhood). Its not wrong taking something existing and spinning/twisting it somehow (like, "what if there was another Maxson-style rebellion, but the Scientists won?), but don't do just that.
(this is a problem with Star Wars as well, btw)
(This is also worth thinking mechanics-wise. My big criticism of Fallout of Nevada is that they were too timid with the mechanics changes. Especially considering how good Nevada is at accomodating diverse skills and stats. Practically didn't touch the SPECIAL system, when a project like theirs was ripe to do so. SPECIAL has its issues, and they have been discussed hundreds of times)
2. Research the local area, not everywhere has to be the West Coast (well, unless it is the West Coast). I feel like Bethesda was mindlessly aping the Classic Fallout aesthetics by having DC be a desert... which literally makes no sense because DC is essentially a giant swamp. The Commonwealth and Appalachia are far more interesting and well-done, in that aspect. Then there's (alternate) history, like "what happened before the war?" and such.
Even the West Coast shouldn't be "just" the West Coast. Central Coast of California and Eastern Nevada are not exactly alike.
3. What's the local myth? This is something I've never seen raised before except by myself. Fallout 1/2/FNV lean strongly in Wild West aesthetics and tropes (yes, folks, Fallout was ALWAYS a western) - 1 already did that but was subtler, 2 was subtle as a club to the head, FNV straight out said "fuck it" and shoved the Wild West in your face, but it was New Vegas, so it worked perfectly. Westerners and Mad Max were some of Fallout's biggest influences.
Fallout 4 seems to lean into the American Revolution, Fallout 76 seems to lean strongly into the old working class struggles of the Appalachian Region (like the Battle of Blair Mountain) and local cryptid and conspiracy theory lore.
It wouldn't make much sense to have cowboys in Porto Rico or Florida, but Pirates? Hell yeah. But Pirates wouldn't make sense in Kansas, if you get my drift.
4. Find the sources of Fallout. Remember, Fallout is a derivative setting. Find these sources. History is always good. Fiction-wise, the Golden Age of Science Fiction is Fallout's big influence. Mad Max as well. There are other non-Golden Age works, like the Hardboiled Graphic Novel.
5. Pre-War is not Post-War. Time passes. The Pre-War world should be this ancient, faded world of yesterday. The Pre-War world ended in the decades after the nukes, and created the "Mad Max Cowboys with Lasers" world of Fallout. People live in the shadow of that great ruin. The world turns, things change. People don't just stay doing the same thing forever. People begin again.
6. Don't contradict previous canon, at least not without a VERY good reason. Like, Super Mutants cannot breed. At all. If there's a Super Mutant breeding, there better be a killer reason for why that is happening, and that should be a massively important plot point - Super Mutants being able to breed would have a bigger impact on the setting than a nuke.