@highvizghilliesuit @atomicpoet @LouisIngenthron @pixelfed I would say it's already happened. Funkwhale allows you to listen to a shared library on the instance. Bookwyrm has book metadata. Calckey and some others have discord like post reactions. We all have our own feature sets, but can still talk a common language of activitypub. If you're on calckey, you could react to this mastodon post using an emote instead of replying to it.
@highvizghilliesuit as long as this hypothetical app is compliant with activitypub, it can federate with mastodon, funkwhale, bookwyrm, calckey, pixelfed, gotosocial, bonfire, write.as, and many more apps that all have their own features.