I was thinking about a much less ambitious design. The way mastodon (activitypub) servers work, global and local feeds are a feature that each server has in-principle available (though the global feed can be individually disabled) and there is no need for a “portal” as such. I imagine discourse could well develop in this direction but it would be a very substantial project.
Stitching together an entry point into the collection of publicly available discourse servers without, e.g. featuring a global feed should be much easier to implement. Come to think of it, it doesnt even have to be a separate project, it could be a plugin that each instance can use to fake a “front page” into the discourse universe. So a rather loosely coupled design which nevertheless might be quite usable.