My top features would be:
- Enable public postings — so that the Fediverse can boost announcements
- Enable full posting of Notes within
[note][/note]up to 500 chars- Maybe add a character counter in the composer (but we can count ourselves)
- Enable federated categories in the sense that a Discourse instance could follow another Discourse instance’s federated category, and users in both forums could discuss.
- Stage Fediverse users
Details
Public postings
Once the main posting feature is a bit more stable, it would be useful for the public to boost posts. E.g., in our use case, the @fep@socialhub.activitypub.rocks posts are new Fediverse Enhancement Proposals, therefore it would be awesome to have a place to advertise them to the Fediverse and hold a conversation there.
Full posting of [note]
Currently the parser cuts too early, and ignores the [note][/note] tags. Since we take time to think about what goes inside the tags, it would be better if the parser would account for good human judgement 
Federated Categories
Bidirectional topics across Discourse instances would certainly mark a milestone in Discourse federation. Although @CodingHorror does not see any use for such a feature, I certainly can, in the sense that many free software communities adopted Discourse for their own usage, and cross-project discussion would add to the power of Discourse forums. See below for the trust issue 
Staged Fediverse Users
When a new user sends an email, it is staged: its account may become a full user account, but in the meantime, it can just participate in the ongoing discussion where it was invited.
The same can be done with Fediverse users: there’s no reason why a Fediverse account could not be staged and follow the trust levels limitations.
Moreover, when a Fediverse account is associated with a Discourse account, the reputation of that account could follow it and transmit the trust… This may open a Pandora box, but within a federation of Discourse instances, it could as well help moderation (against abusive users, and also to facilitate good behavior). This feature alone should de discussed and thought about, as two communities might not give the same credit to the same people. But the bidirectional federated topics would certainly help figuring things out. I would keep that for phase 3 or 4.
Another aspect of staged federated users would be that a person posting from a Mastodon or Pixelfed could then become a Discourse user — i.e., we could have another way of authentication via Fediverse.