I have a feeling that BW and especially B2W2 would've had a lot of features cut from them on the 3DS. None of the 3DS games have come anywhere close to the level of content offered by HGSS and B2W2, and graphical/mechanical limitations are a reason why. Full 3D models require more time and effort and take up more space than sprites, especially on an underpowered console like the 3DS.
For example, a big reason the PWT was possible in B2W2 was because GF could just copy/paste the majority of the battle sprites from Platinum and HGSS, leaving only the Hoenn characters in need of new sprites. On the flipside, SM/USUM couldn't do the same due to using different character models than XY/ORAS (as well as nobody but the Kalos and Hoenn characters even having 3D models in the first place), so we ended up with the watered-down Battle Tree and only a handful of character cameos.