What would it be?
There are two ways to answer a question like this. One would be the most outrageous change you know would never happen, not even in a million years, and one would be a reasonable change. I would say, for the sake of making it simple, that the changes you can make must me single changes. Not complete redesigns.
One Outrageous Change:
The one outrageous change I would like to see, something that will never happen, is for blizzard to adopt an active role in moderating their game. Actively have GMs banning people, actually enforcing the rules that are already in place and making new rules that are meant to make the game a better place for everyone in it, and enforcing them, not pussyfooting around.
It will never happen because blizzard flat out does not give a crap. They ban bots in waves to make their lives easier not to build an anti bot system. Do not buy into the "we need to monitor them so we can stop them" line of bull shit they feed you. If you ban them they are stopped. Isn't that simple enough? They can monitor them for 24 hours and then ban them, that would give them the same data as monitoring them for 2 months, which is roughly how long I have been seeing this same 85 flying around panda land mining and he is still not banned. What information do they get 2 months later after a bot has mined 24/7 for those 2 months that they would not have in the first 24 hours? Once you see what it does, you know what it does, ban them.
I would love to see blizzard actively give a shit about their game and ban the bots, ban the trouble makers, ban anyone that makes the game look bad basically. The quality of game play would increase tenfold if we did not have to deal with all this crap and blizzard turning a blind eye to it. Too bad it will never happen because active moderation means they might lose money because they banned people and money is more important that we, the players, are.
One Easy Change:
For my one easy change I would like to see leveling become a task again. Not meaning hard or anything but I believe it should take a fair deal of time. People should not be capable of getting to max level in 30 hours played if they rush it, it should take 30 days played if they rush it. Maybe 20.
Either way, at the very least change it to the point where even with heirlooms you will not out level a zone 6 quests into it. Make it that even a fully heirloomed player might be able to leave a zone a tiny bit earlier but they would still need to do 70 of the 80 quests in that area to even come close to out leveling it.
People, new people mostly, are missing a fair part of the game. Leveling should be made to be a part of the game again instead of what it is now, a means to an end. If they really want to keep leveling as a means to an end they should just remove the need for leveling all together.
The added side effect is that longer leveling will make the end game of random content so much better. I would hazard a guess that 75% of the players at end game that people might classify as bad players would not be there if it were not for the fact leveling was so fast and easy. It would make the player pool better which in turn would allow for a better end game experience, especially in random content.
There are many things to be gained by making the easy adjustment to leveling speed. There is nothing to be lost. Except subscriber that want everything easy, so I guess even this easy change will never happen.
If you could change one thing, one outrageous and one easy, what would they be?