This is completely subjective, though. KH2 is the pinnacle of the series for you and many people, but for me and other people, it really isn't. That's why KH3 needs to be an independent game that improve its predecessors' strong points and make them better as much as possible. A simple copy and paste of KH2FM's content would have been pointless and sloppy.
First of all, KH3 is the end of a narrative arc, not the end of the series, so it only needs to go all out as far as story is concerned. It doesn't mean that it has an excuse to be a mediocre game gameplay-wise, it just means that what we should expect is indeed "just another KH game" and it's not meant in a bad away at all. A game with 100 hours of story, 20 Disney worlds, 15 original worlds, 15 secret bosses, every little thing from past games implemented into this one would be called "Kingdom Hearts: Opera Omnia". Not Kingdom Hearts 3.
That being said, you're right: we haven't played the game. Not only that, but our only source of information is a dumb leaker that is trolling and deceiving us. This leaker has been playing the game for 2 weeks and he's still not done with it. We know it has side content and worst case scenario, it's the same amount of KH2's side content (which is the game we have to compare KH3 with). To know exactly what the game has in store for us, we have to play it.
You have your personal wishes and there's nothing wrong with that, because everyone has them. You can expect those things in the base game, but it's also completely fine and not that surprising if they are not in the game. KH1, KH2 and BbS have a Final Mix version, why should it be different for KH3? We're getting a full story and side content. It's not like we were deprived of something. Maybe you'll get the boss replay mode in a future DLC or maybe it is already in the game. You can't know.
Eh, this doesn't really make sense to me. I've said it already, everyone has personal wishes. Do you really think that a game (or a book, or a movie) will please EVERY single desire of MILLIONS of players? Of course not. Everyone is bound to some disappointment, it's just natural. I'm disappointed by the lack of FF or because there's no Lilo and Stitch world. Cool, I'm already over it (and I'll get over other things as I play the game). Don't you think players were disappointed by the lack of collectibles in KH2? I'm sure some players must have thought "I wonder what we will be collecting in KH2! There's Stitch, maybe after we visit his world he have to hunt the other experiments!". Not only there was no Lilo and Stitch or experiments, there weren't any collectible whatsoever. This alone didn't cause backlash.
We will experience backlash only if KH3 either doesn't feel like an improvment from KH2 or if it fails at wrapping up the story. I don't care about Data, you do. I care about platform and maybe you don't. None of this will cause backlash. If the story ends with a cliffhanger that revolves around Xehanort or if some important plot points are ignored I'll be pissed. You'll be pissed. Everyone will be pissed and this will definitely cause backlash. Our own wishes differ and KH3 does seem to be offering things for battle lovers, platform lovers, minigames lovers and so on. We only have to wait and see if it delivers.
Just relax and wait for the game. With this mindset, you won't enjoy the game even if it turns out to be almost perfect, that's for certain.