Why didn't the NSDAP tell the Ustase to stop killing Serbs and just treat them as equals?

No idea anything about this stuff. I assume if they were aware of it they just didn't care because they were fighting the war and it's none of their business. They did step in to stop the Iron Guard wiping out a bunch of Jews in Romania, so perhaps the Ustase stuff was either not known about, or the details of it are fudged for propaganda purposes IE both sides were just warcriming the shit out of each other as usual in every war but after the fact only the guys who lost are talked about for obvious reasons.

Also, what exactly would the world likely look like is the Axis won?

If this happened we'd probably not see a one world government of empire or something like that. Instead it'd just be similar to today but with less jewish fuckery. Africa united, probably as a vessel of Europe instead of China. Asia being similar to today, led by Japan instead of China. USA being its own state, Europe being more united maybe including Russia or Russia would be a secondary state like India. The ME would have no Israel-America fuckery, they'd probably be united as a regional bloc under Iran. It would be almost exactly the same as today but with these split-regions being more united and insular. So the ME being more unified rather than being put against each other for the interests of Jews as an example. Of course there would still be differences in neighbour states, and states meddling around the world with each other - intrigue wouldn't go away, but there would be much more regional uniformity and congruence in my opinion, and definitely there would be less spread of liberalism and globohomo which is spread through the power of High Finance.

The goal of Europe would be to unite into a state large enough to sustain itself and defend itself from any other states. This could be a confederation, federation, or single state. I would expect a federation headed by Germany but where the other states have quite a lot of autonomy. Internally Europe would resemble 1933-1938 Germany, characterised by developmentalism, populism/majoritarianism, high culture being made available to the masses, constant standard of living increases, reduction in wealth disparity and unproductive economic practices etc. But this still being balanced with high level of development, usually in the economy ethics and power/development are not aligned. But with careful management of resources it is possible to balance them; profit sharing, capping dividend pay outs, requiring a minimum amount into reinvestment are examples that benefit the economy at large (developmentalism) and the average person (socialism). These are things the Third Reich did, and China do things along these lines today. China is pulling millions of people out of poverty constantly, and growing their middle class. China is basically National Socialism-lite, they are good to look at to see the kind of things that developmental socialist states would aim for.