I've got to the third boss now. I can say a few other things.
The gameplay is pretty much identical to 2 after all. The interface is pretty much a dead ringer. Learning more about stats, AP, DMG, DEF and PP go up a fixed amount every level no matter your partner (1, 4, 3 and 1). Partners affecting stats just seems like a simple + or - to each so Tis who starts out fairly bad might be the best option later when her accuracy cost isn't so bad.
The maps are interesting. They have differing shapes which might be for variety but I get the impression with some gaps (and no hidden walls) that there will be passageways and extra areas in a floor accessed from another floor. I like the sound of that, it's an addition 2 didn't make full use of.
There's more plot. I'm impressed. Potential spoiler, but I might be completely barking up the wrong tree: Fonfon says Fey's scent is very familiar, and she has an almost identical hairstyle to Knight Errant, the remaining boss girl from Cubust's tower who has since vanished. Is it too early to hope?