Currently, talents are the only way to truly customize your characters.
You technically have 5 slots, but one is for healing alone, and another for your elite talent.
So many fun talents! Yet...since you can only pick three, every choice matters. With so little room for choice, efficiency always comes first in the end, and the fun talents are forgotten.
I can't help but think that having one or two more slots for talents would lead to a better experience overall.
One obvious argument against such suggestions always appears, so I'll forestall and address it: "it would make the game harder to balance".
Honestly, the gap in performance would be minor to the point of irrelevance. Besides, no game can be truly balanced, and I don't believe you should be afraid to add depth/choice because of balance concerns alone.
StarCraft II has been out for a decade, it has close to a 50% winrate in all match-ups, and yet people still complain about balance on the forums. Despite that, the devs still make bold changes, like removing units that have been part of the game for years (Mothership core) and introducing new concepts (Shield batteries).
No change can affect balance to the point where the game becomes permanently unplayable.