Credentials: I have a fully incarnated 50 Dark/Dark Controller. Illusion/Rad Controller, Plant/Psi Dominator.
As far as the Dark Control side there are 3 very specific things I think they could adjust/fix that would fine tune Dark Control to make it better/comparable in quality to other top tier control sets (Illusion/Fire/Plant).
1.) Allow another effect to provide Containment. I suggest Fear or Disorient. This would allow Dark to increase it's damage while not changing any current powers and it fits thematically.
2.) Improve Shades. This could be implemented a number of ways. Increasing their attack damage based on a formula of the mob used to cast would be interesting and give a Dark Controller alternative strategy's when casting the spell. (Perhaps adding an additional minor dot based on the "type" of enemy, or increasing attack rate/attack damage) there are a lot of creative and interesting ways this could be accomplished. The goal is to get them somewhat in line with Fire in terms of potential damage/survivability with a small boost. You could even just up the #, adjust the attack rate/damage, etc.
3.) Improve the Umbra Beast. This you could do in a variety of ways. I agree with the poster that mentioned that the howl is too frequent, perhaps changing it to a longer effect/timer so that it is allowed to do more damaging attacks is all it would need. But as it is it does need a bump and with Dark there are a lot of opportunities to adjust it thematically as well as strategically.
If these are adjusted in an upward direction Dark will have a much appreciated boost that fits. I also think this is a low investment in terms of development time. A few minor tweaks and it will compete with the other top tier Controller sets.