Create a CSS Framework, What do I need to know?

What is the problem with current frameworks?

Bootstrap/tailwind?

Yeah. If you would like to make the developer experience better then I recommend you to build a UI kit and sell it as a npm package (you can use https://privjs.com to provide access only to your customers).

For example: The developer should be able to use the UI kit like this:
$ npm i yourawesomeuikit

import { Nav, Hero, Features, Footer } from 'yourawesomeuikit'

export const App = () => (
<>
  <Nav logo="WebsiteLogo" />
  <Hero title="Super Webstie" desc="Website descritpion" />
  <Features list={[...arrayoffeatures]} />
  <Footer copy="copyrights" />
</>
)

I think this would be a dope way of implementing a UI kit

Better idea would be make UI kits from existing free frameworks that already have large community and sell them. The niche is saturated but it does not matter for indie hacker, does it ? :)