I miss GameSpy, the original application, not the service it morphed into later. It was so easy to find a server to play on, playing the levels/mods you wanted to play.
Before that, I spent a lot of time (and money from my dad's credit card) on DWANGO. For those not familiar with DWANGO, you dialed in to their servers and then it acted like you were on a LAN. You could play games like Doom, Doom 2, Duke Nukem 3D, etc against other people. There was a main chat room to talk about what games you wanted to play.
It was also a much nicer place to play, partly because you had to pay _per minute_ in each game. The price wasn't anything crazy, if I recall, but it definitely kept people focused on the game.
Also met some good people and ended up working on a gaming site with one (MeccaWorld.com, on the off chance someone remembers that - I ran the Quake section) and started a company with them a decade or so later.
You drove me down nostalgia lane, looked in email and have my GameSpy receipt with unlock code. We paid $21.55 after tax in March of 1998 - my memory is really fuzzy but besides CS I think Quake III Arena was also popular at the time.
For Lemmy the hub and spoke model is essentially intentional - groups "belong" to a specific instance. But there's nothing in ActivityPub that'd prevent a USENET style model of groups either. There's nothing in ActivityPub that prevetns an application where a collection is effectively open to writing by all, and that would then relay messages to a sufficient set of "downstream" instances.
It'd be interesting to have that as an alternative to the Lemmy approach - I think the two could live quite well side by side.
This reminds me of the design model of SearX/SearXNG - instead of a distributed forge index, it would distribute the search endpoints of forge instances to facilitate the next steps you outline. It almost feels like a central coordinator or maybe a CDN-like network set of search proxies would be needed to do the actual combining and filtering of results. Maybe it could fit in the Codeberg operational umbrella in some future plan.
In practice Nostr does this step on the client side - one subscribes to relays, then when querying for new content it asks all relays, gets all the duplicate metadata and filters on the client. Huge network use and battery drain on your handheld device, Nostr bouncers have emerged for this exact same reason, a popular software is "Bostr", easy to find examples run by random volunteers but it requires money (disk/cpu/ram): https://bostr.azzamo.net/
Screenshot: https://postimg.cc/WFDzQndC
The "subset of the replies" issue likewise isn't inherent, but is somewhat more problematic as it requires everyone to behave in ways that makes it work, e.g. push replies back to the origin server, and regularly poll the origin for additional replies etc., and Mastodon itself is not great at his.
To the extent AcitivityPub itself is affecting any of it, it's only in the sense that ActivityPub imposes very few constraints on implementations, and that leaves a lot of room for specific applications to behave in counter-productive ways..
Mastodon has the similar problem but worse with content discovery; a user is not "seen" remotely by anyone until one remote person finds them and subscribes to their content explicitly. On every single remote instance, which of course is undesirable but that's how ActivityPub is designed.
I don't believe in ForgeFed terms this matters as much as being able to search across the federated network for repos, etc. which I think is a key feature. Sure issues and user accounts and whatnot, but an AP-linked FF-wide search would be insane on how useful it could be for users (and how to implement a "distributed search index" seems like a tough nut to crack).
Codeberg devs have to disable some features (pull mirrors e.g., only push is allowed to prevent abuse) and they use some custom code (abuse mitigation - spam, etc.) but in general you're getting the latest Foregjo experience "test drive" which only gets better when self hosting when you can use all the features.