Integration across various platforms is one of those things where I’m not sure even know whether they want it or not. We’re so used to disconnected platforms that it really is new ground for most.

It’s also probably hard and very open ended.

Which is all to say absolutely, there’d be interest, it’s just that it’ll be a bumpy ride but also worth it for showing people what is possible.

Allowing multiple accounts across multiple platforms to be merged into single timelines in customisable ways, alone, would be awesome.

As far as platforms go, I’d prioritise, on the basis of popularity and uniqueness:

1. Mastodon (for obvious and sad reasons)
2. PeerTube (video is unique on fediverse)
3. Lemmy (communities, link aggregation posts, no character limits, thread management etc are all unique and powerful on the fediverse … could have been 2nd but pushed out IMO because of uniqueness of video).
4. Pixelfed (for the instagram people, but, ASAIK, not too distinct from microblogging, but it is popular, though easily viewed from mastodon I think)

Roll those 4 into one (and maybe even just the top 3) and you would have a banger of an app. Even rolling mastodon and Lemmy into a single interface would be quite a demonstration.