huuduy216 I can't make that particular one open-source due to the licensing agreement indeed.
As I have shared previously, I don't think it's possible to make a free extension that will work for the majority at this point. Supporting all extensions and features is just not possible, and everyone will want different extensions to be supported. Supporting an extension does not necessarily mean allowing guests to use its features. Sometimes if you open up guest posting, it just breaks some of the enabled extensions or creates confusing user experience.
If someone else wants to sponsor me to create a different guest posting extension, I can do it. If the features are similar to those in the existing extension, I'd check with Daniel (who owns the existing extension) if we can extend on the existing code instead of creating a completely new one. Right now I don't have any time for new freelance work unfortunately, so that wouldn't be before a few weeks or months.
One reason this extension is a lot of work is because it has to use Flarum features that are not part of the public API. Even after stable I expect the existing extension will continue to break from time to time as Flarum modifies its internal API. There was an issue on Flarum flarum/core591 (and there might be another) but the consensus was that Flarum wasn't really interested in offering guest posting ability in Flarum itself. Maybe in the future we can make some changes to Flarum's internals to make the third-party guest posting extensions a bit less likely to break.
