@EmeraldCore
In PvE
WoW isn't built for rewarding Stealth-Gameplay. The game is design around 'Combat', and stealth in its very nature is an 'Avoidance of Combat'. Nothing needs to change on the players' end to make [Stealth] fun. The abilities are well designed. But it requires drastic changes in world-building and NPC-behavior code from the Ascension Devs.

Things you need to make [Stealth] exciting:

Make enemies that need to be avoided at all costs, but are positioned in ways that they block progress forward. [Vanish] only drops aggro, but doesn't reset 'non-engaged' monsters. Part of the Stealth-fantasy is opening up, bleeding up a target and vanishing before they can fight back. Problem is that [Vanish] in solo-gameplay resets the enemies, which resets their health. A visual overlay of monsters' perception proximity. Basically a visual indicator of circles and cones you need to avoid stepping in while stealthed. Infiltration and Pick-Pocket quests. A monster mechanic that gives you a debuff of having been 'Detected' and thus failing your quest. Forcing you to go back and 're-accept the quest'. Monsters that when un-CCed for [X]-amount of time call out for help and overwhelm an area with adds. Make it so that [Distract] can halt the adds in place and buy your party more time. Bosses that become more powerful the more enemies were killed before the party comes up to them.

In PvP
It is used as an unreliable gap closer. [Intercept] stuns a target for almost as long as [Cheap Shot]. But one requires point-&-click and the other requires maneuvering yourself slowly up to a target before AoE knocks you out of stealth.

My suggestions for making Stealth better:

Breaking or losing stealth needs to make your untargetable to people outside of melee range for a brief moment. Talents for [Stealth] to make is so that DoT damage does not break your stealth. Change [Master of Subtlety] talent to: "While stealthed and 10 seconds after breaking stealth deal 30% more damage. However, when this damage buff fades you take 30% extra damage for the next 30 seconds." Forcing you to play a Hit and Runner type of PvPer