As time goes on we get more powerful and maybe even surpass the big Warlocks in the WoW universe(like Gul’dan). On that note we should get more powerful and cooler skills 
Right now I have one specific in mind that I would REALLY want.
-FEL EFFLUX-
It could be a Desctruction only spell since it would fit perfecly. Maybe even a charged spell like Fire Breath, the longer you charge it the more ticks/duration with a ramping damage increase to reward longer casts.
Are there any more spells that tickle your fancy?