I seriously wonder why we cannot posit less evil Nazis, but smarter Nazis.
For example, we already know that a bombastic loon can get into power (i.e. Trump.) However, there is nothing inconsistent with a loon being perhaps more pragmatic (i.e. Stalin.)
So, being that the Ukrainians were willing to cooperate and IOTL cooperated quite a bit considering they got totally screwed (kinda shows how much worse Stalin's screwing was as long as you were not Jewish), we can get an interesting POD which can pay big Nazi dividends.
Simply do not arrest Bandera and company right away. Nazi propaganda to their own people stated that it was 1. a pre-emptive strike and 2. a war of liberation for the subjugated peoples of Eastern Europe. So, even if we hear posters point out that Hitler wanted his Lebensraum and bullcrap like that, it's not like Hitler is such a nice guy he would not dare lying to people. The Nazis worked out deals with the Slovaks, Croats, and "mongolized" Hungarians. It is not beyond the realm of possibility that Hitler would work out a deal with the Ukrainians with the same propagandized line. They did that in the Baltic states after all, and long term plans for Ostplan were to depopulate them. So, we already have an OTL example of Hitler playing the short term pragmatist by not killing people that he otherwise planned on killing or expelling from their lands.
This begs the question, so why didn't he? Simple answer: Standard thought in the West during the time was that France was a tougher nut to crack than the Soviet Union. Hitler cracked that nut, so the USSR would be easy. Germany had beat them before in WW1 after all. So, if the USSR is collapsing quicker than terminal velocity, why on Earth not imprison Bandera and other Ukrainian nationalists who will make your job as an occupier harder when you are ready to rape Ukraine.
What we would need is a POD that alters Nazi thinking that a long war with Russia may be necessary. Many here will groan that Hitler only got into short, easy-to-win wars but I call shenanigans on this. Hitler did not think France was going to fall in 6 weeks. He was thinking of a multi-year campaign. So, if Hitler was ready to waste years in France, it is not beyond the realm of possibility he would consider the same for his precious lebensraum.
In short, here is my POD. Finland loses winter war easily. Let's just say that luck goes Russia's way and they steamroll Finland, and annex more Finnish territory (but not the whole country). Now, Russia has hubris and does not confront some of their more systematic problems with their military.
After beating France, Hitler decides that he will have to pre-emptively attack Russia, but plans that it would be a long war. German plans are to break the back of the Soviet military with double-envelopments, so the frontal assaults favored in the Baltics and Ukraine are dumped in favor of a more tactical strategy.
Things go awry when Zhukov gets his way with Stalin's blessing and pre-emptively attacks the Germans on June 10, 1941 with 175 divisions. After making some initial headway, the Soviet colossus bogs down. Hitler declares war and German preparations to pinch off Russian spearheads are surprisingly good and by early August 1941, the front lines are roughly where they were IOTL. During this time, Hitler made guarentees to anti-Soviet, including Bandera, and though things are going well the initial show of Soviet strength intimidates the Germans from being overly-ambitious--especially when they start seeing just how many reserves the Russians have, even after losing so many divisions so quickly.
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Now, if Germany roughly ends 1941 where they did IOTL and are working with the Ukrainians pragmatically, just what good would a liberated Ukraine be? Less partisans I guess, and additional cannon-fodder in the East. I imagine they'd be used to help clear Belarussian partisans and perhaps bolster the Italians, Romanians, and Hungarians after Stalingrad. How big would their numbers be? Perhaps larger than we think, as the Germans captured plenty of Ukrainian soldiers IOTL, they would simply be repatriated to Bandera.
Ultimately, I see Ukrainians as of little practical value when Germany is on the offensive, but from 43-44 they would be a massive thorn in the Soviet Union's side. It might very well butterfly away Bagration, which has huge repercussions, because even another 1.5 million poorly armed Ukrainians is enough of a thorn where the Russians cannot simply allow that large of a military threat to continue.