A Deviantart (or similar art sharing site) clone. None of the current fediverse platforms are really suited for that. Sure you can use Mastodon or PixelFed to post art/fanart, but discovery of both your portfolio and specific pieces is a crapshoot and so is the commenting system for people wanting to give feedback. Also, there's no real way to organize your art like there is with the arbitrary folder structures you can set on proper art sites, so if you're making several pieces in a series, or even more critically, a multi-chapter web comic, you have no way of conveying to your viewers what the groupings and order are.
A Wattpad (or similar story publishing site) clone. Same problem as above. I've tried posting my writings on Lemmy but the relatively low character limit (for a forum) makes it entirely unsuited for long pieces (though this can be pretty easily remedied just by increasing the character limit, long form writings are pretty successful on Reddit and forums after all, see, one, /r/nosleep, /r/worldbuilding and and all the other share-your-story subreddits, and two, all the fanfiction being published and literary roleplays happening on bb style forums. But still, something dedicated to creative writing with built-in support for tags, as well as story partitioning like chapters and series would be ideal.