While I am aware that out-of-the-box Flarum in its aesthetic, function, and design is more geared towards powering tech/general communities rather than roleplaying ones, I still think it would be a worthy feature. I don't expect the official team to take time out of their schedule to implement such a function, so my post is more generally aimed at the third party extension ecosystem. The reason I think a dice roller would be a nice addition is because the market of forums with roleplay-specific functionalities is rather barren, so simply adding a dice roller as an optional extension to Flarum would go a long way. To my knowledge, RPoL is the only free forum with complete roleplaying functionalities. Even then, it is an incredibly antiquated 20 year old website, and it shows both in how it looks and how it functions.
Now, I know that there is an infinite amount of dice rollers out there on the web. When I say "dice roller" I mean specifically what the aforementioned RPoL has: a menu for selecting the dice you want to throw, and displaying all results with notes in a list that everybody on the forum can view (alternatively keeping them secret from users), since the important factor is letting players in the game know roll of dice are authentic.
Possible I could try to develop such an extension myself at some point in the future, but in the meantime I thought I would throw this out here as an idea. Loving Flarum, by the way. Easily best (free) engine to ever come out.