I'll start:
Fallout 3 characters:
Amata: It feels like Bethesda was setting up the potential for a relationship, but ditched it halfway through production. Plus, if you make her the overseer while romanced you can have a speech check to convince her to let you return to your vault.
Moira Brown: Romance could be a long-term reward for helping her write the Wasteland Survival Guide.
Clover: If one has the right amount of caps, they can convince Eulogy Jones to part ways with her. What's done with her then is left up to the player.
Reilly: As a romance option, her character would be extended past her business and her intense quest.
Sierra Petrovita: If one has opted to get Ronald killed with the promise of a threesome, one could then become Sierra's new protector and eventual lover.
Fallout New Vegas characters:
Christine Royce: Cherchez La Femme could do more than get Christine to open up about past relationships.
Arcade: With him having one of the more iconic quest lines in the game, romance with him would be interesting whether or not he stays with the Followers or returns to the Remnants.
Cass: Like Reilly, Cassidy's relations with the courier could be expanded beyond her quest lines.
Veronica Santangelo: It could be a smooth or a rocky romance depending on how the Mojave chapter of the Brotherhood is dealt with.