Im suggesting bunkers of pre-Zday soldiers and military leaders/commanders. Whether a local Nati-Guard unit or a base of power armor wear xenophobes who follow their rules to a letter and dont share their secrets, a base full of mostly hostile NPCs who are offended by your unsolicited presence and can match even the best survivors in terms of armor and short term weapon output.
But yea, references and allusions aside, this would give the Old Guard a kind-of face, open up new missions and perhaps actual merchants with serious stuff worth buying/stealing.