1st - He takes Mettaton's role. Why is it like this?
Story I thought of was some kind of accident with work (sans takes alphys role), which made papyrus lost some part of his body. Sans has built him some parts which made him still be able to live normally. Tho Papyrus lost some or all memories, cuz his head got hurt.
That can sound quite stupid, right?
okay
2nd - He takes Undyne role (it's not about ships!).
He would be the leader of the royal guard. Sans would be quite worried about him tho. That's why we would fight Papyrus and Sans or Sans would join the Battle if it would get intense in the place of Undyne Fight.
We still could visit Papyrus's and Sans house like that and befriend him, but not in the same location probably.
I dunno if anything makes any sense, lol.
I suck....