I don't expect that our choices will have a big impact.
We saw in the SoD trailer that Nilfgaard took over Vizima. So it probably won't matter if Temeria is divided between Radovid/Henselt, a protectorate of Rediana or divided between its barons. Maybe we will see some Redanian/Kaedweni soldiers in Velen or hear people talk about it, depending what happened to Temeria, but nothing more.
About the mages choice: We saw in the 35min demo that there are "Witch Hunters" in Novigrad. As far as I rememberer there weren't any in the previous games, so I assume there will be always a pursuit of mages.
I'm mainly interested in which of the killable characters will return. We know Letho will be back (thanks for that CD Project! one of my favorite characters)
For the others:
Arjan La Valette: Could be back, as we will spent at least 1/3 of the game in Temeria and the La Valette castle is not that far away from Velen and Novigrad.
But I expect rather his mother re-appearing, because she can't die in Witcher 2.
Don't expect more than minor changs, depending if the La Valette Castle was temerian or Nilfgardian at the end of Witcher 2.
Loredo: Not important enough.He dies in 3/4 cases. We won't visit Flotsam
Stennis: We won't visit Aedirn. Doubt he will be back
Henselt: We won't visit Kaedwen(not counting kaer morhen). Doubt he will be back
Saskia. We won't visit the pontar valley. Doubt she will be back:
Sheala: That's difficult. There is definitly unfinished business with her if we let her live. If there will be another returning characters from the ones, that are killable, my bet would be she is one of them.