Dipper and Marcy could definitly be best friends!!!
they have much in common together. They both keep journals, both are adventurous, both like D&D and video games. I can totally see them as BFF’s.
Plus, i’ll bet, that after 3 years (in a alternate universe) they both become a couple, probably because of the same interest and things Dipper and Marcy like ;).
They would be really good friends...but I can imagine Mabel trying to ship them while Sasha and Anne try to explain her that just because two people are good friends doesn't mean they'd make a good couple.