-Dark mode is definitely on top of my list too. I'm using Dark Reader as of now (for both the forum and the main website), but I'd love an official dark mode.
-Log in button on mobile that actually lets me log in instead of randomly opening the Tapas app and nothing else 
-Renaming the "Art | Comics" and "Writing | Novels" into something like "Art Talk" and "Writing Talk", since a lot of the threads in those sections aren't stricly comics and/or novels related but more art and writing related? °°
-A "forum games" section. Here we could move RPs and other types of forum games that are currently in the Off Topic section. Dunno, most forums where I've been usually had the "forum games" section as a separate category, so I thought it'd be nice to have it here too
And now, for the "probably will never happen but one still likes to dream" suggestion...
...Changes to the "Collaborations" category! Namely:
-A marketplace of sorts for artists. This could only be accessed by users who've been on the forums for quite some time. Each artist could have their own thread in which they post their rates and samples of their work (only one thread per artist allowed). A pinned post on top of the category could include artist names listed in alphabetical order with links to their threads.
-A "member feedback" section, in which people who commissioned an artwork can review the service they received (was it fast? did the artist respect deadlines? were prices fair? if physical items were sent, was the shipping good?) and where artist can leave a review of their customer too (was the payment prompt? were the requests clear? and so on). In this way, both artists and potential customers would be able to build a "reputation" based on their previous interactions with other members of the community (if anyone is familiar with the Den Of Angel forums... yes, this is pretty much where I got the whole "member feedback" idea from
).
-A "searching for writer/artist" section. This would be for people who have a very specific request that does not involve a single illustration or art piece and would be more aimed at stuff like webcomic creation and such. Again, users would need to interact on the forum for a while before being able to post in the section... and maybe they could have an actual form to fill with info (again, using Den Of Angels as an example... rather than letting people in the marketplace write their own post, they have a whole form you have to fill with info about the thing you're selling and/or the thing you're looking for).
-A "webcomic collab/art trade" section. This would be strictly for writers/webcomic artists, 100% free and would basically be the place for stuff like Valentine's Day/Pride Month/Halloween collabs, stuff like "I'll draw your character" or even "give me characters to use as extras".