An intermediate win would be arbitrary forms (beyond polls). We support form rendering in our AP objects today, but still waiting for motivated developers to provide a backend layer to provide storage and processing and make it useful. I'm far too busy with other stuff to give this idea the love it deserves.

For apps, I'll just mention what we did with Hubzilla as food for thought. Hubzilla is an access controlled CMS with access-controlled social components.  One of the CMS objects you can create is an 'app' which is nothing more than a potentially access-controlled link with an icon that you can install in your personal channel. It "executes" any web link. Where it gets interesting is that these apps can be accessed-controlled CMS pages created by other Hubzilla members. Or any link you want to provide. And we can share the app in our social stream and let other folks save them and install them on their own channel. I used to use a 3rd party weather app all the time. One of the best examples of this created by members is an online store of goods by visual and audio artists and the link can be restricted to authenticated fans. Oh, and the apps themselves can be paid.

Supporting these objects over ActivityPub isn't that difficult really. It comes down to a rendering of an attached object. We could even do remote storage for your own app collections and let you access them with your fedi-id. Then your personal app manager is nothing more than an app that you can install and share. So as above, so below.  

I think people here lost interest as the fediverse moved to primitive text-based microblogs and away from integrated community tools and ecommerce. Maybe the pendulum will swing back the other way someday.