Easy. Pacifica x Mabel and Dipper x Wendy. Pacifica and Mabel overcoming their differences and eventually dating would be so cute IMO, and Alex Hirsch is super progressive but Disney isn’t, so you never know. Dipper x Wendy for obvious reasons.
If there’s a romance I actually wanted to see that the current storyline would allow, then I definitely want to see Pacifica and Dipper’s relationship be expanded upon, eventually into boyfriend/girlfriend.