The site is very well made I wouldn't change so much.
When you come to the site it gives you a good feeling of "god, I'm inside of the phantom thieves website". Even after being a fanmade one, it is something I wanted to say first.
Second: I feel the site has not so much activity, why is this related to UI? basically is a fact we have to keep in mind if we want the site to still be here and, after all this costs money, better interface can help people stay here and maybe they can help somehow, also, admin will feel more motivated to do changes if the site has activity becasue damn, I will change stuff and people will notice it. Anyways, I feel people discovers the site but they quckly forget about it because... is a bit dead latelly (I'm surprised it is still here, please don't think bad, I'm glad it is still here.), anways, UI is the first thing people see, we have to make it good.
As for the UI I would mostly improve it a bit, maybe give somd additions we don't have or even an APP not only for PMs, maybe to check when people comments in your threads /threads you follow so you can answer quickly or follow a roleplay. Actually, we could try to get more tools to do RP because (speaking from old times) I had to constantly check the site in order to know if people had post or not.
One last thing is the frontal page, basically I hardly ever take a look at it but most of the times is a mess, exactly like the game is NGL, but it can give you an awkard feeling because you can think the people in the forum are like that. Still, maybe we could make it more playable in some way.
As last thing, Royale doesn't change so much as far as I know, but we could still use it to improve the forum, after that even Mishisma in the game says it has a forum. It is so sad it doesn appears in game.
I'll do my best to come up with more ideas, but for now I leave that in case it helps you.
Kind regards.