There is somewhat, but it's more complicated than that. I have been playing the series since I was five years old (when the first game came out), and I wouldn't say that KHII was completely fulfilling. Sure, as a kid I loved it, but that's only because I literally couldn't beat any of the games at the time so the experience never ended for me. However, looking back at it, calling the original fulfilling is not exactly my point of view. I own the original PS2 version still, and there's really no reason to go back to it once you've beaten Xemnas. There's maybe the Ultima Weapon to craft, but other than that, maybe the coliseum cups if you haven't completed them, and Sephiroth, there isn't all that much post-game content. There's no reason to play the original with final mix out, it just beats it out in every conceivable way. New story cutscenes, Limit form, heartless recolors, new cups, Mushroom XIII, Roxas battle, two new keyblades, Cavern of Remembrance & Data Battles, Lingering Will... there is no reason that you'd ever go back to the original.
Kingdom Hearts III at least seems polished where they did do work on it, although I somewhat share the disappointment of area size for the original worlds. I'm probably going to spend way more time in Toy Box and The Caribbean than I ever will in Twilight Town, The Realm of Darkness, or The Keyblade Graveyard.
The plot seems to be a lot more satisfying this time around, however, and that's what I'm looking forward to. Now there's ACTUALLY 13 main villains, there's tons of fan-favorite characters, the organization members seem to be actually tied to the worlds they are assigned to, and the universe has had a lot more time to establish rules this time around. Unlike Kingdom Hearts II, which expected people to play a gameboy advance game to understand the first THREE HOURS of their game, Kingdom Hearts III has made it very clear which games you need to have experience with to understand what they're going for this time. They've even included synopses of plot-important elements so that players can be refreshed on certain games and characters, which is super useful since this game is multi-platform.
Honestly, the biggest missed opportunity was the lack of collection ports to Xbox One. I will never understand that decision.
I know I went on a bit of a tangent, but the point is that I don't know what to expect and that I'm more optimistic than other people here that have stuck with us since the beginning. I still have a huge love and optimism for the franchise, although maybe it's a bit more down-to-earth when it comes to Kingdom Hearts II.