For me, personally the ending of Kingdom Hearts II is just phenomenal. From the music (thanks Shimomura!), to the actual battle design itself, as well as the story concluding, it was perfect. However, due to the masterpiece that was the ending, what can we expect from the ending and how can it top KH2's ending?

KH3 is probably going to end with a more epic battle and a happier ending so I'm not that concerned.

Maybe Sora and Kairi could finally kiss in the ending.

Existing. 

Showing the next villain. Mickey(?)

Usually the games are filled with Sora appreciating his friends. Not that they don't love and appreciate him in return, but I'd like to see all of the surviving members of the team acknowledge Sora as their leader. Maybe one continuous shot of the camera passing every by Riku, Kairi, Mickey, Donald, Goofy, Axel, Roxas, Xion, Terra, Aqua, and Ventus as they look ahead toward Sora who stands a Keyblade Master and protector of all worlds. 

it probably won't

idk...i mean KH2 did a real fine job with the ending. the game had a way of letting you know that "this is it! the final battle! make it count!" the way we revisited old worlds and were given closure on their stories, not to mention that Sora's goal throughout the series (find Riku and bring him back to the islands) was finally accomplished in KH2. Everything was wrapped up neatly in that ending.

I'll have high expectations for KH3's ending. As of right now there's no way it can end on a better note than KH2, especially with all the plot threads it's got to deal with. There's going to be something that just irks me throughout the ending. I don't know what, but it'll be there.

KH2 had Sora and Riku cutting up Xemnas, KH3 will probably have everyone and their mother cutting up Xehanort. It's not hard to one-up tbh