Two things I would like to say, here.
I am -most- concerned with the PvP aspect of Shadow Priests, as opposed to the PvE perspective.
Two things come to mind here, that are -constantly- being discussed (and, my God, the rage that emanates from some of these conversations), that I feel could be slightly changed/fixed, to be tipped in favor of Shadow Priests.
Shadow Orb Generation - yes, it sucks. At least, with one target it does, especially in PvP with interrupts, blanket silences, stuns, general CCs, Line of Sighting, implementing survival, the whole nine yards. My Shadow Priest is specced into "From Darkness, Comes Light" combined with the "Glyph of Mind Spike." It helps a little bit with the interrupts, makes it "safer" to generate Shadow Orbs. All that said, I agree - there needs to be a slight tweak to help Shadow Priests out.
I do have a concern with that, however. Devouring Plague hits like a truck - if we start every fight with a Devouring Plague, that hurts. I wouldn't doubt that, knowing Blizzard, getting to start every fight with an immediate Devouring Plague would mean a nerf to the damage it does, since we'd be "able to use it more."
Mobility - let's face it, Mind Flay is one sucky filler. Woohoo, it slows our target - big freakin' deal. That doesn't help us -at all- if the target is coming after us, and not someone else. So, why not let us move while channeling it? Hell, even if it's only -after- one second of stand-still channeling. Warlocks are going to have Kil'Jaeden's Cunning as a baseline thing, why shouldn't we be able to move when we channel our -only- ability with a slow effect, that doesn't do significant damage, is easily interrupted, and doesn't do anything in regards to generating Shadow Orbs. It's literally a line, between you and your target, that says "interrupt me, please."
Hell, let's take it a step further - why not combine both of these ideas? Scrap the damage on Mind Flay (or modify it to make up for any consequential effects in PvE), and design it to build a stack that lasts 'x' amount of time, and at 'y' stacks gives us a Shadow Orb. If channeled with zero Shadow Orbs, the caster can move while channeling, until at least one Shadow Orb is available to the Shadow Priest. Keep the slow effect.
Now Mind Flay doesn't suck, the slow is worth something, we can still be interrupted, and we don't generate Shadow Orbs at an unreasonably slow rate. If this sounds too overpowered, by all means, modify it as necessary, I don't care. I'm just sick of half of my arena fights consisting of me not getting insta-cast procs, even with all DoTs up on both targets (which is rare, since I usually get bum-rushed), and having to sit on a Mind Flay that doesn't do -anything- for me or my partner. At all. Like, literally, nothing. Right now, as-is, it's a lovely way to get locked out of Shadow spells with interrupts.
That's a very interesting idea. I will have to give it some thought.
The changes made to Devouring Plague limits its burst potential. I do not believe that the shadow orb generation out of combat implementation would make Devouring Plague overpowered, especially in the current state of facemelting on the PTR..