Personally I love it when seemingly inconsequential characters from the previous games get elevated into important roles (like Isabella in DA2 and Samson in DAI). Its fun glue to hold the franchise together, but it also gives the writers a lot of freedom to flesh the character out since their isn't a lot of narrative baggage to sort through. With that in mind, Scout Harding would be my first choice, hopefully as a romance option so that dwarf players can finally have one (I'm rooting for you folks, I really am).
Of the playable characters, my choices would be Dorian, Bull, or Sera. I doubt the fact that each is a potential LI will have much impact on a decision to include them moving forward. Morrigan and Liliana both had significant roles in DAI. They find ways to write around that sort of thing.
As for who I could live without: I think I'm ready to move on for Liliana in any major role. I like her a lot, but both in game and in my own head, her story feels complete to me. Its time for her and my warden to ride off into the sunset, with a few passing references and nothing more. As Varric my put it, they're getting too old for this ****.