Ever since I found out about Onslaught I couldn't shake off the idea of him as a more literal child of Magneto and Professor X. Where, instead of being a big-ass armored dude who's basically a force of nature from the beginning, he starts out as a baby and has to grow into his powers.
Like, how would that even work? Imagine, you are a psychic entity who is made out of all the hatred and fear and despair of two VERY powerful mutants... but you are also a child. You don't know sh&t. How would your nature affect your mental health and your perception of things around you? What kind of behavioral and mental issues can a person have if they are made out of someone else's doom and gloom? Will you start remembering your parents trauma as you grow up?
You also have two not very good dads with different opinions on how to deal with the world, who probably won't really know how to deal with you now that you exist. How do you raise someone made out of your own angst? You are also a living reminder of a VERY messed up incident on top of that.
X-Men and the Brotherhood probably know what you're made of and how you came to be, which will definitely affect the way they think of you. And you can read their minds. This will probably make things really awkward.
And then imagine if humans find out that you exist. Specifically those who hate mutants. It's the hight of the anti-mutant hysteria, your dad just turned off the fucking planet. In their eyes you are a mix of two very influential and very powerful mutants - a prime hostage material and a huge future "threat", regardless of your moral standing or the fact that you are also a fucking child.
Ultimately, I feel like baby Onslaught would be a pretty interesting Nature vs Nurture debate subject. His character would probably go to the Hell and back as he goes from one writer to another, but I kinda want to see that story and the kind of person that he could grow up into.
P. S. If it wasn't clear I'm an optimist. I don't really want "he's evil cuz he was made of evil" kinda shtick. Grey morality? Sure. But I also want to see him deal with these demons that he inherited.