Support for Twitter Polls (which look incredible and should honestly also make Twitter, Inc. ashamed.)
New app icons (Tapbots: please put out a public call for new branding. I guarantee the community would come through.)
and more subjectively…
a more reliable Twitter client than Twitter’s own. For the record, both Tweetbot 6 and Tweetbot 5 have been demonstrably more reliable for me in the past few months than Twitter’s native app, which has been crashing multiple times per day on my devices for quite a while during regular use.