...With the rest of Europe weakened, who would stop the Soviets?
Their own inertia.
Seriously,I believe that de facto, Stalin would be a stabilizing force, unless someone tried to organize an attack against him.
The various Leninist regimes that have developed the world over don't have much of a track record of adventurous conquest. The Russians got entangled in Afghanistan, a little country peripheral to the Western World save as a buffer state, and it was like catching their beards in a buzzsaw. Mao did swallow up Tibet and there were sporadic flare ups with India. One can see the Korean war as joint proxy aggression of both Stalin and Mao--but if you get into examining recently exposed Soviet archival evidence, one can see that actually Stalin viewed Mao as a dangerous loose cannon stirring up trouble where he didn't want it, and Mao had a similar view of the North Koreans they were sponsoring. Also during the Cuban Missile Crisis, Castro proved far more gung-ho about risking a nuclear blowup than the Soviets--not only did the Russians pull out the IRBMs the Americans knew about, they also yanked some tactical nukes out of Cuban hands that the West never knew about until some Russians told Western scholars at a conference in the 1990s.
In general, Leninist regimes, and the Soviets in particular, were very risk-averse. This conflicted with their rhetoric of world revolution, but when push came to shove, Leninists generally preferred to stick with ruling what they already had control over rather than risking it in the hope of getting more. In Stalin's case--it wasn't that he didn't want successful Communist revolutions in places like Germany and China, but he did want to have control over these regimes, and purging the leaders he couldn't rely on and imposing directives on the local Parties that were more aimed at Stalin's preoccupations as ruler of the USSR than advancing toward revolutionary victory in those countries pretty much precluded these parties from taking power. Whenever Leninist Communism did advance, it was either under the aegis of the Red Army advancing against Hitler (and the Japanese, in the endgame) or it was a victory of loose cannons like Mao Zedong, Ho Chi Minh, Fidel Castro, who came to power pretty much because the Soviets had no effective leverage in their settings whatsoever.
So--would Stalin have launched an attack on Eastern Europe unprovoked? I imagine he would have spent his life building up a military machine with that plan in mind--then tear it apart again every 7 or 8 years or so to purge it for disloyalty. I doubt he'd ever get around to actually launching the attack unless some hothead regime with more guts than brains struck first. Perhaps Stalin would arrange for that to happen--more likely unwittingly though, by sponsoring some revolutionary party in Poland or Rumania or Hungary that gets out of hand (and this might happen more than once and true to form, Stalin would move to suppress them himself lest they get too provocative, but once they might move faster than he could stop them) and the reactionaries that crush them decide they'd better strike at the Red Menace in its homeland.
But with Britain and France holding the upper hand against Hitler, with Hitler replaced in Germany by some other kind of dictator and curbed from developing sufficient power to threaten the West, the Germans would perforce lack the power to threaten the Soviets either, even if they cobbled together a sweeping alliance of all Eastern European regimes against Russia. The likely result would be stalemate, and that's just how Britain, as well as Stalin and his successors, would want it. Even if the Germans did organize a Mitteleuropean coalition, given these great-power realities the coalition would probably serve mainly to restrain its members from doing anything rash.