What's left of the USSR will be much less powerful. It's economy probably 25%+ smaller. This state is still a great power, but hardly comparable to the USSR of OTL. Ukraine, fertile fields and the Donbas basin, contains significant amounts of everything, natural resources, population, industry, and perhaps most importantly, food needed to fuel a superpower. IOTL, between 1917 and 1948 there were several famines in the Soviet Union, of course with varying causes for each one. The famines after the end of the war will be much worse without Ukrainian grain. If the government remains committed to collectivization, which they most likely must be if the Bolsheviks are in charge, I could see the 30s being much hungrier in both the cities and the countryside of Russia proper. We know of grain warehouses containing rotting foodstuffs in the cities in 1933 confiscated from Ukrainian farmers, without that food, feeding Soviet cities gets much harder.
As to central Asia, what we will see first isn't the "virgin lands campaign" but de-nomadization, aka Genocide of Central Asian peoples. As in OTL, the hope was to turn nomads into farmers and then from there, as sedentary people, into perfect communist societies. What resulted was atrocious death, Kazakhs lost ~38-42% of their population or ~1.5-2.2 million people. This is new research, since it was taboo in Kazakhstan until very very recently, it's still taboo in much of the rest of Central Asia, so it's harder for researchers to figure out, but it's likely other parts of Central Asia also suffered. If the Soviets try to turn Northern Kazakhstan into an ersatz breadbasket, they likely will start with de-nomadization, and when that doesn't work, have to push in Russian farmers. OTL during both the Imperial and Soviet periods many Ukrainians settled in northern Kazakhstan, southern Siberia, and the Far East and were pretty important in their development, likewise industrially those regions were far less industrialized before WWII as well.
I would guess that collectivization remains deadly even without the Ukrainian SSR as a field for genocide. Undoubtedly famines in Russia will be worse than IOTL without Ukrainian grain to confiscate. This could cause other problems, the Soviet modus operandi was in times of famine to just confiscate at gunpoint food in rural areas and bring it to the cities, the peasants being a disposable class in the Communist (and especially Marxist-Leninist) system.
What's interesting, is that with the Soviet Union further afield from Weimar Germany, maybe they will instead seek a military relationship with Ukraine and not the Soviet Union. Obviously, during WWI Germany supported the Skoropads'kyi regime, after Riga, Germany and Austria were a hotbed of Ukrainian exiles seeking to organize expeditions to invade Soviet occupied Ukraine. These were funded by diehard anti-communist businessmen in exchange for investment in a future liberated Ukraine. Some of these plans went further than you might think, before the UNR in-exile in Poland tried something similar and mucked it all up, scaring away many supporters. Of course, those plans would probably have failed as well, but it goes to show links between not only Imperial Germany as is widely known, but also Weimar Germany. This is an interesting possibility, Ukraine had a quarter of the USSR's heavy industry, and if Ukraine has Eastern Galicia, it also has oil, so it's a natural fit. Poland is an obvious mutual rival, if Czechoslovakia doesn't give Ukraine Zakarpattia it, likewise, will be a mutual rival, although I would imagine they probably would allow it to join Ukraine since that was the most popular vision for the region, and it was only due to pressure from Woodrow Wilson and Polish successes in the field of battle. Although what kind of government Ukraine has will effect this, although quite likely is a Democratic Socialist one, aligning with the German government before 1933. Likewise it's military is probably fairly right wing, so gets along with the Heer. If Ukraine wins independence, I'm assuming that Petliurist purges of the military do not happen. The execution of Col. Bolbochan and the firing of Oleksandr Hrekiv/Hrekov are the most significant examples of this short-sighted policy.
If Germany still signs Molotov-Ribbentrop, they probably retain Lithuania and much more territory in Poland, if not basically all of it, pending their Eastern borders ITTL. Indeed, perhaps, even if the Nazis and Stalin come to power ITTL, they do not see the need for the agreement at all, as the Soviet Union will be interested in expansion probably in the Caucasus first, outside the German area of immediate interest. Without Weimar-USSR agreements maybe the foundation is simply not there.
To determine Poland's relationship with Ukraine and Belarus, we'd have to get into specifics of how they achieve independence. Does the Polish-Ukrainian alliance win? Are the National-Democrats defeated, allowing Piłsudski to put a promethean in charge of negotiating at an alt-Riga following increased military successes? It's often forgotten that Poland was offered basically all of Belarus, including Minsk, as well as much of Central Ukraine, and they told the USSR to keep the territory since the National Democrats in charge of negotiating at Riga felt that they could not absorb too many non-Poles, otherwise assimilation and colonization of the East would be too difficult.
