I can't imagine a spin off, but if in chapter 4 we get to play as others for awhile (I imagine playing as Morgan very briefly while Guybrush is in jail) it'd be alot of fun. Imagine what would be in her pockets?
"It's my assasins ledger. Handy for any bounty hunter. Guybrush put it back inside the wrong boot."
Fun things like that make me smile, look how cool it was to check the Voodoo lady's pockets for random things.
"I have such fond memories of that blade held to my throat. Ah, those were good times." - Guybrush Threepwood on Morgan's Sword