Content wise, I'm hoping for something more than Shadowfell Conspiracy and closer to Menace of the Underdark. I feel like we have lots of classes and races. I'd mainly like to see more content, items, raids, and in particular, new art. The typical series of 3 quests and a series cap quest seems a bit long per sitting. Maybe have more series of 3 quests.
I do like the idea some others have mentioned where we would get some new series, and some older series would be updated. I think many of the original low level quests make the game look bad to new players. If it was up to me I would either update them or replace them over time.
One item I would really like to see is avatar updates. Better art and animation would put a fresher face on the game. It would also be nice to have some limited control on height and width. I can imagine there are many challenges with making existing items fit, but maybe some smart people there can figure out a method to auto-scale. Maybe that could tie into a better overall armor look, instead of having pieces attached to a skin. Also give us more color. Almost all armor has drab, faded, and dingy colors.
Adding some new weapon types could be nice too. That is one thing we really haven't seen changed since the game started.
Wilderness areas - I don't feel like we get the right bang for the development buck with them currently. Except for some areas that scale mobs with party size, slayer XP isn't great. I don't see many slayer LFMs except for the scaling areas and we know that is a laggy mechanic. Explorers are interesting once but XP isn't great and the map benefit is lost with a TR/IR, so the incentive for the time seems low. Same with rares. Most the time they are pretty backgrounds used to run to a quest, that you hope you can eventually teleport to. Think of some ways to make wilderness areas more desirable. Maybe a method to collect crafting materials in rare chests? The named items in 3BC made that area popular for a while. Explorers give access to teleport points or shortcuts? Favor for slayers?