Assuming that Russia and China were willing to give up that land (after all, they aren't very nationalistic, aggressive in seeking and retaining territory, have powerful armies, nor are they ruthless towards minorities), I don't see how it'd work from a demographics perspective.
Inner Mongolia in China has ethnic Mongols in the minority. Unless I've done my sums wrong, if the areas in Inner Mongolia, present-day Mongolia, plus the Mongolian parts of Russia were combined (one possible pan-Mongolian state mentioned in the Wikipedia article), Han Chinese would form a majority, leaving ethnic Mongols in a minority: Inner Mongolia had 19,650,687 Han Chinese out of 24,706,321, Tuva has a total population of 307,930, the Altay Republic has a total population of 206,168, Buryatia has a total population of 972,021, and present day Mongolia has a total population of 3,081,677.
As far as I can tell, it's not possible to be one state, predominantly controlled by ethnic Mongols, and democratic. (Kind of like John Kerry's comments about Israel) What approaches have present-day pan-Mongolians advocated for this demographic issue?