While I have suggestions for both Blorp and Mlem that I hope will ultimately lead to them adopting more Arctic features (a shame it wasn't open sourced), the basic feature I think would be most useful are swipe actions. I think I mentioned it at some point, but the option to swipe as an alternative to tapping small icons would improve accessibility and versatility (ideally such swipes could be customized). Arctic's implementation is still my favorite, but as another reference, Mlem did update their implementation to also feature a customizable three-swipe per side system.

Something else I think would be useful would be the addition of an 'All' section to the left of 'Posts' and 'Comments' on the user profile page as a way of seeing both content types grouped together by overall recency.

Of the listed features, I think having an alternative to Interstellar with Lemmy, Piefed, and Mbin support would be great, but in pragmatic terms the small user base for Mbin might warrant focusing efforts elsewhere first. I think better in-app content support would be a good point of differentiation.