I don't feel like I should have a chance at saying what would make Affliction better as I haven't played it but let's see about Demo and especially Destro:
Demo:
1) Make Demonic Empowerment have 2 charges like conflag, also instant. Each Demonic Empowerment gives an empower of 15 seconds. CD is 20 seconds. Empowerment now increases your character's shadow damage done against your target by 5% for its duration. It's called Demonic Empowerment, not Demon Empowerment. Why not get a demonic buff along with your minions? Also change the animation to look much cooler.
2) Make it so that Summon Dreadstalkers becomes an instant cast if you use hand of gul'dan before it.
3) Make demonwrath a respectable aoe attack. Or make implosion baseline and add something else in that row. Or make your demons spit hellfire, similar to how the old hellfire aoe was for warlock demon form. Make it so that your demons just emit hellfire through an instant cast. Similar to the demonic form that you would just press the ability and it would work for 10 secs or something.
4) Doomguard and Infernal should become part of a regular rotation with 1.5 min cd. You should be able to summon either, you are a mastery demonologist for christ's sake. Also, dmg of these 2 summons would be reduced to compensate for less cd and being able to use them both.
5) Give dreadstalkers the ability to blink towards the target you shadow/demonbolt too. THat way they will lose much less time running and doing nothing.
6) New Demo 3-4 min cd. Summon Pit Lord. Make it powerful. A combination of the infernal and doomguard, as well as with new abilities. 25 second duration if 3 min cd. 30 second duration if 4 min cd. Make this cd synergize with the two demon legendaries.
7) No fucking mana issues.
Ok, these might have been some silly changes, I don't know, not a demo expert, its just my offspecc! Now destro!
1) Reverse Entropy baseline.
2) Cataclysm baseline, make its old talent an improved Cataclysm that summons an infernal too or something.
3) New Fel Animations.
4) Make the Magistrike Restraints legendary to hit the same target if it duplicates a chaos bolt and no other target is nearby. I hate it how its just a tiny stack stick in pure ST fights. I don't have the legendary yet by the way but I really like the idea of a duplicated chaos bolt.
5) Rework mastery. If you want a chaotic mastery I will give you one. Every attack you use has a 1-X% chance depending on your mastery to give you a free chaos bolt. Instant cast. 100% Increased damage. Also Mastery won't be such a ridiculous % per mastery now. Its so stupid to see my warlock have 38% mastery as demo and then 78% as destro lol.
6) Due to the rework of mastery, you will obviously not have that 'average' dmg increase from regular chaotic energies. Say 35% more average dmg during a fight if you have 70% mastery. This is why all spells would be buffed. Most of the spells would get a small 5-10% increase in damage. Chaos bolt would get 40-50% damage increase. The reason I thought of this mastery is because I tried to think what's really fun for me in the game. Then I thought fire mages, and their instant cast pyroblasts with MASSIVE damage. Then I tried to think of what us Destro liked from our specc, and that was Chaos bolts - the gigantic amount of dmg they dealt back in the day. So I tried to create instant Chaos bolts, and creating a new mastery for destro that would make sense as 'chaotic', if not 'chaotically destructive'.
This would rape people in pvp so for this, blizz would make it so that the extra chaos bolt wouldn't do 100% increased damage NOR would it benefit from Wrought chaos.
7) Dark Soul CD: Straight dmg cd. We need one other than doomguard/infernal.
8) Lots of ppl will suggest Doomguard and Infernal having no summoning cost but I think it fits.
9) Rework Grimoire of Supremacy into giving you a much more powerful demon that would be just as good of a choice as Grimoire of Service, so that the Supremacy demon's dps will be just as good over a 3 minute fight as summoning 2 bursty imps with grimoire of service. Make it a new cool demon that uses fire and its dmg scales with immolate slightly better. (Can't think of any specific one at the moment). It really sucks that Destro does the most dmg it can do with a pathetic imp... lol.
I would like to be given the choice to either use the pathetic imp on the double or a new cool demon that deals just as much dmg as the two imps.
For all: Enslave Demon, because hunters!
EDIT: Sorry if my English is a bit butchered but I just woke up haha! Also I would like to mention that even though I would love all these changes, I am still pretty happy with destro especially. The only things I really want are cata to be baseline since I never pick it, or reverse entropy since then I could most likely pick cata from the talent row. And another thing I hate is the mastery. It's so weird and stupid. Nice idea, but I wish it wasn't my favourite specc that has it. >_> The whole fantasy is chaotic destructive, this is why the specc is called Destruction, but its so sad that my Chaos bolts sometimes hit 700k on pve enemies, but sometimes 300k... Makes me feel like my class fantasy is 'Chaotic Puppy'.