The Brotherhood Outcasts would've been a really nice faction in Fo4, people who didn't like Elder Maxson or folks that helped Synths/married to Synths etc.
Would've given another faction to roleplay with, a mix between the strict BoS and the Railroad (which imo can't "reign" the commonwealth or even make it a better place when some time passed).
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Nuka Cola. Nuka Cola never changes.