I think the biggest issue facing defilers is the question, "why should we use a defiler when we can use a mystic?" Mystics heal nearly as good as defilers, dps significantly better than defilers and bring tons of utilty to the raid (stampede, bolster anyone?)
In order for defilers to compete we need to do something better than our shammy brothers. The obvious answer would be to say lets give defilers greater healing power. Maybe another group ward or something. But the current raid content (even in POW) doesn't really require additional healing than what a defiler already brings. Its the same issue that faced templars, raid mobs don't require so much healing that there is a detriment to bringing a mystic over a defiler.
The next possibilty would be to increase the defiler's dps. Now this could be possible. Make it so that defilers continue to excel in the AE dps arena. Another blue ae (maybe with significant debuffs attached) would be cool. The new prestige that adds an extra strike to wrath has promise (though as others said it should probably be imprecate or another defiler specific ability). But I think you could take it even a step further. What about make it like the warlock's flames of velious enhancement. Give it an additional strike at every 25% increment of the mobs health. For example 1 extra hit after the mob hits 75%, 2 after 50% and 3 extra hits (4 in total) for the final 25%. You could also tag additional debuffs to each strike so flavorwise the defiler is sucking the soul out of the mob the closer it is to death. Another way to boost defilers dps is to give them an additional 20% SDA on their self wisdom buff (can't recall the name).
The final (and I would say best way to help defilers) would be to increase their utility. Here are some of my thoughts:
1. Bring back deadly bane warding as a prestige effect. I see alot of other classes getting procs or what not that attribute the damage to themselves. Would be cool to have an effect like this that attributes the damage back to the defiler (though pre-warding may require a tank to be more careful on pulls so the defiler doesn't go splat by the initial agro build).
2. Give us some kind of stoneskin or 100% Damage immunity for like 5 seconds. Defilers are typically in the maintank group. Give defilers a save on a base 5 minute timer so it gives the defiler a chance to help out with a face melting ae on the tank here and there. To make it different from other blocks, it could be for the whole group. Its not often enough that it infringes on other healers but gives a bit of extra flexibility for those "oh crap everything is down aes" Its also has a short duration so it would test the skill and timing of the defiler to use properly.
3. Add some effects to ROA. I am thinking, a decent boost of CB and on any hit, the person hitting the mob will get a chance to proce a ward.
4. Make Harbringer groupwide. Gives the group more hps and a ward (assuming the defiler takes the focus).
5. Have a prestige that turns spiritual circle into a true regenerating AE ward. Make it affect up to 12 people closest to the defiler with a max range of like 30m. Alternatively, you could keep spiritual circle as it is, but make it immune to ae and give the whole raid CB and Potency while its up.
6. Have an ability that is kind of like RO/VC that monitors the amount of damage that is warded over a 10s time period then at the conclusion it automatically gives the group a damage proc based on that amount of damage.