Three that I find intriguing and would fit in the game.

Arcane Trickster - Others have stated this, and I echo it as well. Some might say you can sort of make one already with the Feydark tree. I mean, kind of but not really. It has been quite a while since Rogues got a look. Granted multiclass versions of them are popular with Horizon Walker and Inquisitive but that also speaks to the versatility of the Rogue class. Putting in Arcane Trickster and designing it so there will be synergy with the casting perks from Shadowdancer seems like a golden opportunity.

Shadow Caster - Sorcerer is another class that hasn't been given a look in quite a while. More or less, we are still stuck with the pick an elemental color and go with it class. Shadow Caster would bring exciting new spells and utility to the class. Shadow Caster is another archetype that could benefit from some existing game mechanics in the Shadowdancer destiny requiring less Dev design type. Hound of Ill Omen introduction could also lay the framework for a future Ranger archetype Beastmaster. Finally, Shadow Form - and please make sure we can float like the Wraith form from Pale Master!

Path of Wild Magic - Perhaps we should just embrace that casting has advantages and offer that option on Barbarian. The Path of Wild Magic would be like Eldrich Knight, but Barbarian style. This would be all about the Wild Surge and unstable magics around you. Think of a playable version of the Scion of Limbo.