Rosa/Rydia! Post-game, with Cecil and Rosa having an open relationship (maybe before Rydia and Edge get hitched, or maybe Edge is involved too), maybe with cultural meta about relationships. As for plot, umm, I would love some politics with Rydia as the de facto leader of Mist and the representative of the eidolons, and how her relationship with Rosa conflicts with (or enhances!) her political role. Or possibly an attempt to recapture Rydia's ability to use white magic (which can go well, or not).