Assassin and AP Bruiser aren't even the same thing
Assassins are get in > kill > get out before you get killed or kill something else in theory. Diana has almost no escape mobility to speak of.
A Bruiser, which covers both Divers (Vi, Diana, Irelia) and some Skirmishers (Yasuo, Fiora, Jax), usually doesnt kill with the rapidness and efficiency of an Assassin, but they command a kill-or-be-killed presence that draws attention towards themselves as they can kill anything in their path if given the time to, and they have tankiness and/or defensive capabilities to live through the attention they demand from the enemy.
The difference is Divers are tankier and have easier access to the backline, but deal slightly less damage than Skirmishers, who have higher sustained damage and more open mobility with defensive tools to help them survive instead of innate tankiness.
And if we're being honest here, "Bruiser" refers more to the build path than a kit-style-- one that focuses on a balance between damage and tankiness for an intended effect. Juggernauts aren't Bruisers because they take both to the extreme, not "balance", but are unforgivably immobile and are vulnerable to kiting. Diana does tend to build more damage than your usual Diver.
As someone who played a lot of Diana in my earlier days of league, I would welcome a Katarina-level update. Something to pull away from the pretty linear playstyle, and to be frank, it's slightly overdue for such an amazing champion.