TYPO3 Core

What is the reason that you want this? Normal visitors don't see this sitemap. It is only for devs / SEOs to have an easy overview.

When I created this, I also thought about overrides but couldn't come up with a valid use-case for this.

... to change default title and headline "TYPO3 XML Sitemap". My customer want to have "My Company XML Sitemap".

It is already possible to override the XML templates by settings the templateRootPaths. You can add your own XSL to that file if you want. Does that solve your issue? I think there is quite a small amount of people that would like to override this XSL file.

overwriting templateRootPaths does not effect Sitemap.xsl

See https://gist.github.com/haassie/6fb1086dcb98f7532af45c6fa083c9fe for an example how to do this. In the example only the index is changes, but you can also do this for your other Sitemap.xml