I agree that it’s extremely important to provide a stellar new user experience, and that the current fediverse is severely lacking in this regard, but I’m not sure it should be in the form of a bot. I’ve seen all kinds of things shoehorned into an instant messaging UI in Telegram, and they don’t always work well in that format. It’s the same concern here over making the welcome UI based on posts, as those are about the only common feature among all existing AP implementations.
Besides, how would a bot remind someone about something? Using a privately addressed Note? That won’t work with Smithereen as I intentionally reject anything non-public. How would it explain privacy settings if Smithereen has none? Also Smithereen doesn’t have pinned posts. And hashtags. IMO, it’d be better if every implementation would create its own onboarding experience that best suits its intended use and feature set. As with all universal abstractions, the bot would either have to use very vague phrasing to suit all possible UIs, or be too confusing if your server software isn’t about microblogging.
Then there’s the problem of centralization. The bot will be hosted on some instance, and, in a sense, the rest of the fediverse would depend on that instance, thus somewhat defeating the purpose of decentralization.
The one thing that doesn’t seem to be addressed by anyone is the discovery process of the people you already know. Mastodon claims to solve discovery by having public feeds (instance and global), but those only help you see random people you don’t know — which is not what most people want. I can’t repeat this enough, but it’s absolutely crucial to have some form of (decentralized) global search that would at least allow associating user IDs in centralized social media to ActivityPub IDs. I’m thinking about using something like a DHT (distributed hash table) made out of instance servers, but this has never advanced past my thoughts and so is still open for debate. This alone would go a long way to solve the dreaded “I’ve signed up, now what” problem.