No other Pokemon will be getting new formes aside from Kyurem.
Take a look at this
article.
This lists all Pokemon found in the BW coding that has different formes, including non-legendaries.
If suppose there were other Pokemon that will be getting new formes in BW2, it should be part of this list.
How is that proof of no other Pokemon getting forms? Can that bit of coding not be changed for BW2? Besides, that list indicates that Kyurem will be limited to two forms; Kyurem is already confirmed to have three.
I believe that Kyurem will actually have four forms. Call me crazy, but I could see Reshiram and Zekrom getting new forms as well. Hear me out: according to Unova mythology, Reshiram and Zekrom were once a single Pokemon that was broken into two; it is indicated in BW1 that the form of Kyurem available for capture is the "shell" of of that Pokemon. That looks to be the case as well; Reshiram and Zekrom stand tall and gracefully, while Kyurem looks to be old and broken. If this myth has any truth to it, it is logical to believe that Kyurem was once a whole being that channelled both fire and lightning energies, and that these energies were stripped from it when Reshiram and Zekrom were concieved. No energy means no heat, and results in ice (which explains Kyurem's typing and bodily makeup).
That said, Kyurem's new Black and White forms seem to convey that in BW2, Kyurem will absorb back half of the energy that its whole form once had. That is why each form is half-covered in ice: because half of its energy is still missing.
So if the energy is "absorbed back" into Kyurem to create these forms, where does that leave Reshiram and Zekrom? Well, obviously they're not going to disappear for many obvious reasons regarding game mechanics, so I think they're going to simply be "drained" of their energy similar to how Kyurem's standard form is drained. This is where I could see Reshiram and Zekrom each having new forms: "drained" forms that seem old and decrepit like Kyurem's standard form.
It's a bit of a stretch, but it fits what we know and what we've seen.