When growing up in Vault 101 you have your best friend Amata. While growing up you have the various options to help Amata, now, what angered me is that the goodbye scene at the Vault door could of been so much more, it was hinted that a male wanderer had feelings for Amata but this was never fulfilled. Even worse during the return to Vault 101 there is NO emotional impact at all! the wanderer simply "doesn't care" which is what annoys me the most.
Then, at the end of Broken Steel, its pretty obvious that Sarah Lyons has feelings for the wanderer, yet this is not branched out on at all, frustrating. Also there are many, MANY, more characters in Fallout 3 that scream romance sub-plot potential such as Sydney or Butch.
And, unlike New Vegas, Fallout 3 had no mods that added to this at all.. the lone wanderer was always well, alone.
Now, lets move onto New Vegas... the one time Bethesda got companion character development right! sort of.. although I'd go as far to say that was Obsidians influence.
I mean, they had romance sub-plot potential there to rival Mass Effect! They had Cass and Arcade for Males and Veronica and Boone for Females, hell, you could even say Raul would of had potential, I mean, his story is one of the best things in New Vegas!
Again, it flirted with the idea and nothing came of it.
Now, Willow came out for PC, the best companion mod in New Vegas and when Willow came out.. it was kind of a proof of concept that Romance sub-plots DO work well in open world games and can be entirely optional at the same time.
Now, lets go to Skyrim.
So much potential especially with the Marriage system, however, we all know Skyrim's marriage is a sham. It makes no sense at all! there is no romance there no character development.. its pathetic..
Now, lets fast forward to Dawnguard and look as Serana.. I mean come on, SO MUCH wasted potential there.. it was pretty obvious that a romance sub-plot could of easily been put there. Serana is not just popular because of her rack but because of her character, one of the best written Bethesda characters ever and they do not expand on it at all.
Again, it was up to a modder to prove that Romance subplots can work in an open world game and be entirely optional.. Vilja is possibly the most greatest mod for Skyrim simply because it offers this.
So, my main point is, in Fallout 4 or the Next Elder Scrolls.. we need to have a proper romantic sub plot, honestly, it is a good idea.