The account ownership one is definitely my first thought too. Using keypairs to identify accounts controlled by the user would be great. You can hide it behind a normal looking username with your (or a third party’s) domain on both Nostr and AT (though AT’s not fully developed, but it’s totally in control of the user and doing that with activity pub could hand the metaphorical keys of your account to you instead of the server. Another way would be to allow users to use their own domain with an existing third party server, or use something similar to Zot’s (Hubzilla) “Nomadic Identities” that let you mirror your account on another server.
Implementing quote posts is a little above my pay grade, but I actually think that the link option might be a great idea. Nostr does quote posts by just embedding the post into your post where the nevent is pasted in, making both regular quote posts and the general ability to embed any post anywhere in your post. That sort of functionality, just using links to posts instead of nevents could be neat to standardize into activity pub as well, especially if all the other activity pub software already is doing that. Though again, not exactly a dev, so who knows if that’d break things.