I gave it some thought, but this WOULD be possible with the main cast if the Devs do choose to "retire" them.

If it's relegated to a fun side quest series, and the characters only pop in the MSQ here and there, that would mean the devs could give us those extra romantic events without it potentially damaging or severely limiting the main story or becoming too complex with having to write so many extra layers of text or cutscenes if you're hitched with a fellow Scion or former leader character.

But honestly I would expect small things like, if you hook up with Urianger, you find him hanging in your inn rooms with a few items scattered around that are his stuff. Like a telescope, and some astronomy charts. He'd have a few bits of dialogue reflecting current events if he's found there. When you hop in bed he climbs in, too, and when you wake up he gets out with you.

That kind of stuff. The kind of thing you usually start seeing in the sim games that have relationships.