A quick note on possible improvements that I would love,
Some sort of horde mode. It would be a lot of fun and might give people another reason to log on.
What I had in mind was something like an endless waves of monsters that increase in density and type/power, engaged after taking down a new boss/uber or even existing bosses/ubers that ends when everyone in the party dies or even when the last man is left standing on the end game Diablo encounter. This could encourage boss encounters since the possibility of Diablo or other bosses actually dropping something makes fighting them more appealing. And multilayer play is encouraged if the second (end game) option is implemented.
From a visual perspective it would be nice to have an option to edit how legendaries look that goes further than dyes. Perhaps the ability to go into an editor where you can adjust sliders to change colors of items, the cost being a brim and a drop from a new boss encounter? maybe a silly one put into whimsyshire??. If OLD games like armored core had the ability to personalize avatars there is no reason why a game like diablo cannot incorporate such a feature. Even the option to change a characters skin color/hairstyle would be amusing.