Give him more credit, I'll give you a few ideas, some outlandish and some make more sense.
1. Jill/Bella combined path, sure this will take significant setup, but didn't we get a hint that something happened between them when they slept together in bed, before Jill left and joined you in the sofa? If you go Bella it could make sense pretty easily, imo, and probably on the other end, to.
2. Same can be said on Sage/Jm, we KNOW there's attraction there, and we know J/M is ok with a relationship bigger than 2, Sage just seems open to it, as long as it is on the up and up.
3. The "others" route, ofc, with maybe a drunk slip up that leads to fuck buddies with some of the previous main LI's
4. Others route with many girls, like you said.
I'm no writer, and i thoughts of those in second, I'm choosing to believe DPC has similar or better plans.