Well, considering that none of the KH games so far had a true happy ending (KH 2 may count for those fans who still think that Roxas and Naminé don't deserve to be their own entities and disregard the peril of TAV), I would welcome a true happy ending after all the sadness and sacrifices resulting in fates worse than death in the games before.
The bittersweet note in the end could probably related somehow to Isa, as while Terra fought against Xehanort's influence from the beginning, Isa certainly went along with it for a while.
So it may come to the conclusion that while Isa does get redeemed due to Lea's efforts, unlike Terra he doesn't manage to get all the way back and perishes together with MX, Ansem SoD and Xemnas for good.
OR it could relate to the fact that there needs to be something done in order to ensure that Xehanort stays gone, and Isa (possibly alongside Eraqus) decides to play the role of keeping Xehanort gone forever, which requires him to not return with the others.
Lea gaining back Roxas and Xion but effectively losing Isa in the process would be certainly a bittersweet jab.
My stance on romance may or may not be already widely known around the forum, but I'd say again that I prefer it ambigious and largely left out of the main canon. It's just annoying and detracts from the main friendship/companion theme.
As for the outlook for the next saga, that should probably be reserved for the secret ending, possibly even in a FMV-minddiddly scene again like after KH 1 and KH 2 which makes everyone go WTF instead of including it in the regular ending.
The regular ending should possibly involve a sort of "victory party" of the good guys (probably a beach party at Destiny Islands (or Twilight Town, to include Hayner and co.), letting Roxas and Xion finally get to the beach) giving truly some sort of closure with the main trios returning to their home/main worlds during the credits, meaning MDG back to Disney Castle, TAV to Land of Departure, SRK to Destiny Islands and RAX + Naminé to Twilight Town (or Naminé stays on DI as well).