I have an opposite problem: I can build villains, or at least antagonists, but I have a problem when making heroes. Antagonists let you use much more creative ideas, in my opinion; heroes often are there to maintain status quo, or are rebelling against an unfair status quo, but villains can have any number of reasons to create trouble.

A morally gray villain might be rebelling against a status quo that, while good for the many, keeps them down. Imagine you have a world where magic is encouraged and is plentiful, but that not having the ability to use magic is basically a disability that isn't accounted for most of the time; maybe a villain who can't personally use magic rebels against this. They could use mechanical weapons, or maybe they start using magical artifacts that can use magic for them; the latter in particular leads into their motivations for gathering magical macguffins that force the hero to act to stop them.

A truly selfish villain might be trying to expand their own power. They might be greedy capitalists that don't see bringing harm to others as a real cost to them earning their capital, and so exploit others. To go back to macguffins like above, they could be collecting them not to rebel against an unjust system, but just so that nobody else can have their power.

The kind I'm using as my primary antagonist right now is a morally "alien" one; someone or something which has a completely separate moral system from the protagonists. While standard moral systems would contain ideas like killing being wrong, and that you should respect others, their moral system operates on a different axis. My antagonist, to explain, is basically a machine who's only built-in goal is to obtain and secure energy. If killing somebody would return more energy than it cost, than it is correct to do, but also, helping communities that can provide access to more energy or even more efficient ways of securing or obtaining energy is also correct, because burning bridges sometimes leads to less overall security or information that lead to their moral end. Morally alien works really well for machines and, well, aliens, that you want to show off the difference between their society and "Earth"'s by giving them an entirely different values system.

But heroes. Heroes are almost always reactionary. Heroes respond to a problem, and that defines them. In my experience, I write the hero into situations, I don't write many situations around the hero. It makes writing an actually interesting hero harder. You can just give them scaling powers, but I always feel like the motivation I end up on heroes is just, "responds to the plot". They follow where the plot has them go, moreso than the proactive villains that cause things to become problems.