I'm not sure about new features, but I do hope to see the Spin-dash to return.

I missed that since the first paper Mario.

I'd like to see more badges that change the player's approach in playing, kinda like what the slow badge did in TTYD and the first game.

Yeah, I was really happy that they brought back turn-based battles, but I'm waiting in hope for badges to return.
I'm also really hoping to see some new abilities of Paper Mario like there were in TTYD like how Mario could turn into a paper airplane, boat, etc.

Fully Orchestrated soundtrack
none of this midi cause "its a portable blah blah blah" bullshit

Maybe the true forms of Blumiere and Timpani somewhere, like in a picture.

^they might put an Easter egg or something like that in PM3DS, that usually what they do, but that all depends which time period this game takes place in, if they keep it linear or *crag* up the timeline like they did Zelda, Metroid, and tons of other games.

Way technicaaaaaaaal bosses  and good partners