Toriel and Sans
On pen and paper, it might be a decent idea, but not when it actually happens.
1. Toriel has the personaility of a mother and would most likely treat Sans like a cute little son that makes her laugh at the things he says.
2. Sans would laugh at the idea of those two being together, and would love to hang out with her. But not to the extent of falling in love with her and MARRYING HER.
3. She might go back to Asgore, it'll take time and a second chance.
Undyne and Alphys
Look, they're more than likely going to stay as BFF's cause of they're time together. I mean, all power to Undyne from making Alphys prevent suicide by making her comfortable. And I perfectly understand if she respects Undyne because of that, but if anything, I'd say she would need Mettaton to be her love interest. She's a scientist, and science comes first! Besides, Papyrus could be better for Undyne if he studied more on dating. Yes, I believe Papyrus and Undyne are better than Alphys and Undyne together.
Papyrus and Mettaton
Who was on drugs when somebody invented this? Its beyond stupid, its beyond reasoning, its just insanity.
1. Mettaton cares only for his fans and his fame. Without it, he has nothing. Maybe Alphys at the same time.
2. He wouldn't know who Papyrus is and just brushes him off as a fan.
3. That leg thing in the end was the cause of this pairing, people are just crazy by inventing things that aren't even there.
Other pairings like Frisk and Asriel, Chara and Asriel...just stop. Stop pretending thay nearly every character in this game and many others somehow have a love connection and claim they exist because you say so.