[@WanderingDragon] 
I think you mix up several issues and present them as one. Let’s break down them both, shall we?

Three things you must ask yourself:
1. What is more important: polyamorous plot or this exact set of characters?
2. Would you return a favor of playing a set of characters that somebody else developed to fulfill their dream plot? And wouldn’t you feel used and exploited like a sextoy in process?
3. Why would you even start playing with somebody else, if you’re fixated on your set of characters? Isn’t it settling for less?
Answer honestly, and contemplate why other people are wary of such a proposal. I know I used to pick up roles written by someone else, and those unanimously left me with ashen taste in mouth, because my partner cared more for her token baes and little for my own ideas.

However.
As a game master or a player leading a good fat plot line, a cast of well-developed supporting characters around my main character, NPCs played for partners here and there and otherwise, for me is a good practice of ages. Of course I mostly play my set of kids/exes/friends and business partners and my partners keep theirs and we exchange roles only after permission. Polyamory is tricky, it doesn’t work for majority of people because people are selfish, and the same applies on player’s personal level. You have to employ people who are more CRPG-minded rather than fanfiction/literature oriented, because I see there a deep rift in cognition and deep bonding of player’s self with their character, borderline self insert. The same unsettling fixation on partner/partner’s character tend to be present, too, so beware.