Attribution:. If your fork retains a significant proportion of the original code then just leave the existing authors in the metadata. You can simply add yourself. In themes or theme complaints that’s usually in about.json. In plugins this is in plugin.rb. It’s also good to mention them in the README.md if you like.
Often plugins, themes and theme components have dedicated topics here on meta. If so you can thank them there, or if you create a topic for your distinct fork you can credit the original authors there as a footnote in the OP.
If you publish a supporting Topic the general community expectation will be that you will maintain the asset over years so bear that in mind. It is best not to publish a Topic if you don’t intend to maintain compatibility with evolving Discourse or provide users some level of support. My advice here would be build it first then decide later if you want to publicly support it.