based on the screenshots, i'd say it will probably stay more or less the same. there may be a few changes here and there, but i think it will probably stick to the classic paper mario formula. besides, I think that our palls at Nint10d0h know 1 thing: if it ain't broke, don't fix it.
"Yeah! For what it's worth, I'm willing to unleash the power of the pants!" -Quote by Neemon, in Digimon Frontier episode 43
Its already pretty similar to both Mario and Luigi and Mario RPG. You've got the whole timing / specific action to power up attacks, timing to avoid attacks or reduce damage, special moves (each game taking a slightly different route with them, but still more or less the same in general), ect. All of the major battle mechanics are pretty similar between the 3.
There are slight differences, of course. Mario and Luigi has 2 set characters, Paper Mario has 1 set character and 1 changable character, and Mario RPG has 1 set and 2 changable. Mario and Luigi focuses on outright avoiding damage altogether, where the others are more reduction (aside from Superguarding). Mario RPG gives all characters a set of specials, Paper Mario gives both characters their own standard attacks (some of which could be considered specials) and gives Mario a list of actual specials. Mario and Luigi gives you a set of standard commands, and combined specials. But those are all pretty minor.
The only big difference is the perspective, where Paper Mario is on a flat plane, Mario and Luigi is on a right to left 3D plane, and Mario RPG is on a top to bottom 3D plane.
I'm not really sure how it could become more similar without just outright adopting the exact battle system of one of the other games.
But anyway, I hope Paper Mario stays just as it is (or was in the first 2 games). And by the look of the screens, its more or less going to.