Create a website (app store) that utilizes GitHub (also GitLab?) API to search for repos that have a webxdc or webxdc-app tag (topic): webxdc · GitHub Topics · GitHub. (I’m in favor of the latter, because webxdc may also include tools related to webxdc).
Why not just link to GitHub directly? Because the UI is not that convenient, there’s no direct download link, and a lot of other clutter (for people who just want to download the app).
How do you post an app to an app store? Just create a repo, add description, and a release with the .xdc file attached. Also works with closed-source apps.
I think given the amount of possible filters (by author, by number of stars, update date) it makes sense to search for an existing “Advanced GitHub search” website (or at least a query-composing library). Yes, the native GitHub website advanced search looks bad for this task.
Also perhaps as long as there are not too many webxdc apps, filtering/sorting can be done client-side.
IMO one important thing about the app store(s) is that it should be easy for people to feel it before they have to install anything. A web-based app store is a solution to this.
E.g. F-Droid also has a web version.