i think i would like to see stuff like galleries and writing threads more organised into sections, a bit more "gallery" style than post,post,post. so you can view all a users art etc, and comment on an individual piece rather than searching through posts and commenting on bunches. i mean that makes it significantly different from a forum (and oddly similar to deviant art), as coryn rightly noted, thats what we actually are.
notifications would be fantastic for messages and replies, new threads etc.
to be honest its been in my head for a while. ive been getting into app coding more recency, but have wanted to make this for ages.
UI wise, i would expect it to be similar to the website itself with the sections etc, but easier to read definitely. plus the idea in my first paragraph.
to be honest, a mobile skin or .css would be much more achievable. some extensions etc allow you to do it personally anyway, like forcing text to be a certain size or colour. ive never done it personally but i know its around.