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Bill Cipher/Fiddleford McGucket/Stanford Pines:

(Spoilers for Gravity Falls, Journal 3, and The Book of Bill)

It's just a very interesting dynamic. Fiddleford and Ford were college roommates, wink wink nudge nudge, and then live together in a secluded cabin that Fiddleford left his wife and young child to go live in because Ford simply asked for his help. They're best friends but there's a lot to suggest that at the MINIMUM, Fiddleford certainly has feelings for Ford. (Forgetting to get his wife a Christmas gift but making two for Ford, making Ford's name his password, etc.)

Bill and Ford are ambiguously canon. It's certainly up to interpretation how exactly you read their relationship but at the very least, it's blatant both of them loved each other. (Ford literally worshipping Bill for over a year, giving him full permission to possess him as he pleased, calling himself a Cipherholic, etc. Mabel telling Bill to go crush on somebody else's uncle, "one Sixer, please", "if lost return to Bill", etc.) Whether they ever voiced that to each other in a romantic sense, unclear.

And then there's Fiddleford and Bill. Fiddleford says he never met him, but he also wiped his own memory hundreds of times so he's not the most reliable narrator. Ford thinks he was hiding Bill's existence well, but as you can see in the attached image, he was NOT subtle about his worshipping, even letting himself be possessed in front of Fiddleford. Fiddleford's invention leads to Bill's death. They're both in love with the same man, and are both too emotionally constipated to do anything useful about it. Their potential interactions are a relatively untapped treasure trove of possibility.

Shen Qingqiu(Shen Yuan)/Luo Binghe/Liu Qingge:

Luo Binghe and Shen Qingqiu are married by the end of the series and have both been in sort-of courtships with Liu Qingge for years but they’re all such oblivious idiots that none of them realize this.