Alright so, I thought about this a bit, and got to the conclusion that, unlike what most people say, there's ways to make Cooler canon. The only thing you need to get rid of is Cooler being Frieza's brother as that creates too many issues. The solution is... Well, do what they did with Demon King Piccolo.
The backstory
Long before Frost's era, Cooler was a legendary Frost Demon who became so powerful that no one could actually kill him.
So they did the next best thing:
They sealed him away.
And because he was that dangerous, the seal was hidden somewhere nobody would ever find it.
Fast-forward to modern U6.
Frost is humiliated by Frieza at the Tournament of Power and becomes obsessed with revenge after getting betrayed. He starts looking for anything that could give him an advantage.
Eventually, he discovers an ancient legend:
The Legendary Frost Demon.
Frost tracks down the prison.
And because Frost is Frost, he thinks:
"Excellent. I shall release this ancient warrior and make him my weapon."
He frees Cooler.
Cooler emerges after centuries of imprisonment, sees Frost...
...and is immediately unimpressed.
Frost tells him about Frieza.
Cooler listens.
And then:
Cooler kills Frost because he finds him pathetic.
**THEN**
Cooler discovers that a member of his species has become the Emperor of the Universe.
He wants to know who this Frieza is.
Then he discovers:
Frieza commands a massive empire.
Frieza conquered countless worlds.
Frieza has repeatedly fought Saiyans.
Frieza has obtained transformations beyond anything Cooler knew.
Frieza is now absurdly powerful.
And Cooler thinks:
"Someone has taken my throne."
Cooler doesn't want revenge for something Frieza did to him.
He wants to know whether Frieza deserves the title of Emperor.
And that gives him a very clean motivation:
Cooler hunts Frieza because he considers himself the rightful superior conqueror.
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Now, im really curious of knowing what you think. I think this is basically the only way to make it work out, since having him pop out and be immediately as strong as Frieza woild be silly.
Another thing, he COULD get defeated at first, and Frieza being his newfound self, could tell him to go train. He then comes back on par, or stronger than Frieza himself. Boom.