I cannot speak for the community but as a pvp-person who has been doing only that for about 5 years I'm going to list my suggestions on how to make pvp better as a whole. I will start with the less obvious and uncommon suggestions and end with the common ones (since they got posted a few times already).
Balance around 3v3, but keep 2v2 and especially 1v1 in mind.
I have read many posts where players get frustrated about a single ability or talent. One of the most popular ones was maybe Second Wind. In every post people would call it overpowered and others would try to counter that argument with saying that it’s balanced in 3v3.
The thing is… both of them are right. Second Wind is way more powerful in a 1v1 situation than in a 3v3 situation. People will always have 1on1 fights, (especially) more so than 3v3 fights. Keep in mind that not everyone does 3v3 arena, some players might only like BGs and duels or they simply don’t have time to find 2 other players every time they want to do pvp.
Make defensive cooldown usage proactive instead of reactive.
This means that a dk would have to use IBF when he anticipates a stun instead of doing it only when he got stunned. The problem with reactive cooldowns is that it makes the game a lot easier. Furthermore pvp becomes something like a binairy fight where you can almost simulate what is going to happen. Proactive cooldown usage would promote skillful play and makes fights less predictable and more interesting.
Less damage from (static) damage sources that don't really require the players attention.
Examples of these are usually pets (Wild Imps, BM hunter pets) and procs like Cobra Strikes (makes the hunter pet do critical strikes for over 20k damage easily without any awareness from the hunter). I think it’s fairly self-explanatory as to why these shouldn’t have a big influence on the outcome of a pvp battle.
Just because an ability looks or feels cool and gets a lot of player praise doesn’t mean it shouldn't apply to the balancing rules.
Even if the players like it, in the grand scheme of pvp it will do more harm than good if it’s overpowered. (Example: Avatar at the start of MoP)
Keep RNG but reduce it’s influence.
Generally as a pvp’er I don’t like RNG. But I agree that it has a place in pvp and it’s sometimes fun to have back to back procs/ RNG luck. However this element should be kept at a minimum because it’s based on luck and nothing else. An example of RNG that is too strong in some cases is Paralytic poison. A rogue might just put you into a Garrote followed by a 6 second kidney shot for 10 seconds and get a fourth application of the Paralytic poison at the end making it a 14 second lockdown scenario compared to a 10 second one. It’s all just a game of luck and rewards the player for attacking, not for playing smart.
Removal of abilities with multiple charges.
I feel like Blizzard has gone overboard with the system of abilities having charges. I see a few problems with them. The first problem is that it becomes very hard to track or memorize enemy ability usage. Let’s say we gave all abilities 3 charges, would you be able to keep track of them? The second problem is back-to-back usage of these abilities which makes it hard to balance around. An example of this is the double charge system of warriors. When balancing warriors with a single charge of Charge you could use the logic that when the warrior charges, the opponent can get away with some form of escaping tool and now the warrior cant attack for *insert number here* seconds. Based on this you can get a general idea of warrior uptime which means you can balance their damage appropriately. As soon as you bring a double charge into play things get messed up. Some classes with a limited escape mechanism toolkit won’t ever be able to get away while other classes still can. This makes a very big difference in terms of uptime and makes for a less balanced system. Shifting example ( A = class with plate and one tool to escape, B = class with cloth and multiple tools to escape).
Single charge system: Uptime on A = 70%, Uptime on B = 20% (difference 50%)
Double charge system: Uptime on A = 95%, Uptime on B = 25% (difference 70%)
As you can see the difference becomes bigger which creates problems that can only be solved by giving class A (dull) passive damage reduction buffs (Focused Will, Glyph of Lightning Shield,…) or other band-aid fixes that have a further negative impact on pvp balance in general.
Diminishing returns on cc reworked
In the current state of the game I feel like diminishing returns on cc should work differently. It’s always one type of cc followed by another and into another. While I could suggest a different DR system I don’t think it would be relevant for 6.0. It’s up to Blizzard to think of a DR system that is appropriate to how CC works in the next expansion.
No more blankets on interrupts.
Shortly said it rewards bad play and punishes the better player (person who managed to fake cast successfully) to have these combined in one spell. If you insist that a class needs a silence AND an interrupt you should just give them a shared cooldown. You could for example make counterspell an interrupt only that interrupts for 6 seconds and you could give them a silence that silences for 3 seconds. However, if either one gets used the other one should go on cooldown as well. You could even make one trigger a shorter cooldown than the other (like shaman shocks).
Promote casting, reduce influence of instant casts
While the cooldowns on interrupts got increased I still feel like it didn’t help too much with the “promote casting” thing. Only when instant cc gets reduced to a much lower amount casting can become viable again.
Another factor is also that instant casts are usually more powerful that casted spells. Be it Ice Lance or instant Lava Bursts, it’s all instant and it’s a lot of damage. Getting a Lava Burst proc is like doing 2 casted lightning bolts.
Lastly mind numbing effects have to be removed. There is actually no question here, if you intend to let it stay it should be 5 or 10% at maximum.
Make instant cc a niche, don't hand it out to everyone.
I think many would agree that instant cc in MoP got out of hand. This doesn't mean that all of it should be removed. In my opinion it would be better to have it as a niche, like how HoJ was always instant for paladins but they don't have a slow or MS effect.
Bring back uniqueness for each class.
It’s something we all crave.
Offensive cooldown stacking removed or diminished
Right now I think everyone is using a macro with all their offensive cooldowns listed in it. I don’t think that’s what Blizzard intended with offensive cooldows because it’s rather stupid (ME PRESS 1 BUTTON FOR HIGH DMG!!!).
Now this doesn’t only apply to class specific abilities, but also damage buffs from trinkets. (I like the pvp on use trinket by itself, but not paired/macroed with any offensive cooldown).
Increase the cooldown of offensive and defensive cooldowns.
Generally when I’m fighting in battlegrounds I have most of my cooldowns available in every other fight. It just feels like I have to use them all the time because they will be off cooldown in the next fight anyways. Maybe they have received a lower cooldown because most abilities have become reactive abilities(as explained earlier).
Make classes easy to learn, but hard to master.
However the game turns out to be, I believe this is the most important one to keep in mind at all times. Reward those who have learned to be a better player. If it means that one very good player can kill 2 bad players then so be it. Don’t have it like the current state where one of the bad players can “outplay” you with the press of a dps burst macro.
These were the suggestions that I could come up with at the moment. WoW pvp can be great but steps have to be taken and I'm hoping for a better game in 6.0.