I'd like to see change in game mechanics, but I worry of course about the risk of changing damaging the game.
I'd like to see change in the client. I like the cartoony graphics, but I'd like to see them offering more character customization - LoneStar style face generation, for example, not just the same half dozen faces.
I'd like to see non-game-mechanic changes in the client - for example, I'd like to see improvements in the Auction House. I'd like to be able to have orderings by price be for the whole set of an item for sale, not just the current page; I'd like to be able to filter more (only show auctions for full packs of cloth, to avoid the people who post pages and pages of one cloth for sale).
Most importantly and fundamentally, I'd like to see changes in the economy and the items NPCs sell. Vendors basically are useless to buy from - they almost never sell anything worthwhile, and when they do, it's crazy money. I'd like to see the economy become genuinely meaningful - which means, for example, NPCs only buying stuff they can actually then sell at a profit to other players (so for one part of this, they'd buy anything to begin with, but if they have some in stock, they won't buy it any more).
I'd like to see better player crafting, too. Rather than the fixed set of greens and blues, allow players (e.g. blacksmiths for weapons) generate a budget for an item, where that budget comes from the value and rareness of the materials used and the skill of the BS, and then decide himself how that budget is spent on bonuses (which of course are not just +n stat, but also procs), so he actually designs a custom weapon.
This would also create a market for player crafted goods - right now, almost all player receipes are not worth making.
I want to see ring crafting, too, since it's missing - everything else can be crafted. None of this jewel rubbish though; that's different. An item already by itself allows any stats or procs, so jewels (as I understand them - I never played retail with jewels) add nothing new. Rings however are a item which currently is only available through drops. It seems odd players can craft most things, but just not rings.
Also, I'd like to see players be able to craft mundane items, like vials. You'd need mats, but you could make them, so you could sell them - all good for the player economy and the creation of wealth.