Mongolia (both inner and outer), Manchuria and East Turkistan can all be seized given that they all border Russia.
I could see Qinghai and Gansu, and maybe even Tibet being annexed, depending on how far they can push their luck vis a vis other powers.
And how much of it can be held on indefinitively, like Primorsky?
Theoretically, all of it, if Russia (a) settles the area with enough colonists, (b) grabs it early enough, (c) is determined enough, and (d) is stable enough to hold them.