You can skip my explanation if you don't really care. This is a long post, I know.
DoT classes traditionally have two strengths: their ability to continue a higher level of DPS while moving/incapacitated/turned away and the ability to DPS multiple targets simultaneously. In turn their weaknesses have been scaling issues and long setup time/low burst; hopefully the former will be resolved by the 3.3 changes. Since theoretically DPS should be roughly equal among classes (as always a topic of vehement debate, I won't be going into it here), what you come down to are the comparative strengths and weaknesses of DPS styles.
Many fights emphasize rapid target switching as a major source of DPS challenge. Due to DoT classes having a longer ramp-up time, this leaves them at a disadvantage if the target reliably dies in under ~30 seconds. Well, this isn't necessarily a terrible thing if DoTs are able to shine under other circumstances; after all, if all classes were equally effective on all fights then there wouldn't be much of a difference between them.
My concern however is that the strengths of DoT classes don't tend to be very impressive; that is not to say that I don't enjoy the playstyle, but is there any specific benefit to this approach (this is a more pressing question for Warlocks who have both DoT and burst specs)? Being able to DPS when other classes are unable to is convenient, but doesn't seem to shine for most fights (Yogg being an exception). Multi-DoTing as a source of DPS increase is a topic of much debate; it's not that it isn't a good DPS boost to the Priest, but whether it's useful to the raid is foggier. That is, which is a better situation: to have two mobs at half health, or to have one alive and one dead? Under the majority of circumstances it's better to kill one target quickly than to DPS several equally, and so unless you are struggling with a berserk timer and this truly ups raid success multi-DoTing can be a questionable strategy.
And with this in mind, I'd like to suggest an ability that props up the functionality of multi-DoTing as well as assist with target-switching. There are two separate versions I have in mind with different functionalities. In both versions they function as the present Beacon of Light does with regards to the affected target: that is, the DoTs on the Beaconed target aren't included, just DoTs on nearby targets. Also, I'm just throwing numbers out so don't waste your time critiquing whether the numbers are balanced; it's the discussion of the idea I'm after.
Beacon of Darkness v1 - Instant
You place a Beacon of Darkness on the target for five minutes, causing it to take 25% of the damage dealt by your Shadow Word: Pain, Vampiric Touch, and Devouring Plague abilities to up to two targets within 40 yards. If any targets within 40 yards are afflicted with Shadow Word: Pain or Vampiric Touch then the Beaconed target will be treated similarly.
This is the version that most closely mirrors the ability's namesake. It would be a near-permanent debuff on the target that would essentially justify DoTing multiple targets, as it would be a DPS boost to your primary target (probably the boss) to DoT up secondary ones (probably adds). It also offers a mechanic similar to that of Pestilence: after having set up your DoTs (diseases) on a target somewhere, you can import that benefit to a secondary target. I don't want to remove the initial ramp up of DoT classes, but I do want to soften subsequent switches.
My biggest concern is that this would create balancing issues for S.priests if it were too strong. That is, either S.priests would have to be sub-par DPS on single-target fights and count on multi-DoTing to be worthwhile, or they'd be grossly over effective where Beacon of Darkness were applicable. I've tried to keep the numbers down so that it more counteracts the single-target DPS loss of multi-DoTing than greatly increase overall DPS.
Beacon of Darkness v2 - 1.5 Second Cast
You place a Beacon of Darkness on the target for 30 seconds, causing it to take 200% of the damage dealt by your Shadow Word: Pain, Vampiric Touch, and Devouring Plague abilities to all targets within 40 yards. If any targets within 40 yards are afflicted with Shadow Word: Pain or Vampiric Touch then the Beaconed target will be treated similarly. 2 minute cooldown.
This is a different version in that rather than being a constant DPS boost it's a burst cooldown. It isn't like Haunt where it buffs your DoTs on the main target, but instead a cooldown that rewards setup as well as giving you the ability to assist with adds more effectively.
In summary, I think that at present the basic benefits of DoTs (uninterrupted DPS and multi-DoTing) are not entirely outweighed on most fights by the lack of burst damage and target switching potential. I don't want to give S.priests massive burst so that they're just see-through Destruction Warlocks, but I do want to help accentuate their strengths and shore up the weaknesses slightly.