My main is a shadow tank. I spend most of my time focused on the rows of buff icons trying to catch the moment when harnessed shadows procs so I can fire off the next project. In ops where there are often 3 rows of icons coming and going, it is difficult to see when double strike crits to give me harnessed shadows.
This could be solved by adding an element to the UI where a select few buff states are in fixed positions (such as just above the right end of the player health bar but below the debuffs). These would be greyed out when not in effect and light up when they activate. It would only need a few slots - say 3 to 5 - but it would allow more attention to be paid to what's going on in the rest of the fight instead of having to focus on the icons so much. The same is true for other classes too.
If it can be moved around as an independent UI element, that would be even better. Healers could move it over next to the group health bars for example.
I imagine the simplest way for it to work would be to bind the effect you want in each slot from a drop-down list of possible effects (possibly categorized by type or source such as class effects, group long term buffs, group short term buffs, adrenals, stims, medpacs, relics). Drag & drop would be a pain since you'd only be able to set it up while in combat. Another option might be have them filled automatically and only cover skill tree (or advanced class if any) proc effects which are no doubt the most important ones to watch for. I suspect that might cut down on the coding required to implement it.