Much of this is probaly going to echo what other people have already said, but here we go anyway.
I like the Captain and have it as my main character, but the class has its problems.
A couple of recent(ish) changes to the Captain really reduced QoL.
* The new 5 minute duration on Motivating Speech is highly annoying. You have to keep remembering to refresh it from time to time during longer combats.
* The reduction in range for Noble Mark sucks. It is one of the few ranged abilities Captains have, and was very useful for pulling mobs. It was apparently reduced due to being considered too good or something - but I have not heard of any player thinking it was too good. On the contrary, Noble Mark is widely considered the weakest of the Mark abilities.
It would be nice if the Yellow line had a "big" opener attack like Shadow's Lament in Red or Valiant Strike in Blue.
Tanking captains have a hard time keeping aggro against multiple foes due to the low number of AoE abilities. Would be nice if there were at least more ways of generating AoE aggro, even if not AoE damage.
I would love for Captains to be able to use bows just like Guardians or Champions. Useful for pulling, and useful for doing at least some damage with auto-attacks when (for whatever reason) you are unable to engage in melee. I also would like it for purely aesthetic reasons.
(Now someone will say something about our banners already taking up the ranged weapon slot. I have an idea for that, but it is probably trickier to implement than it sounds: Make captains have banners as their Legendary Item instead of emblems (adjusting stats on banners as appropriate). That way you free up the ranged slot for bows. Banners are kind of iconic for captains, unlike emblems.)
Somewhat improved heralds would be nice. Especially the Archer who is supposed to do more damage than the other Heralds, but the difference in damage is too small to matter.