I feel that MathOverflow is unique in the StackExchange network. There are features which we might want which might be unique to this site, and which might not be immediate feature requests or requests that StackExchange would want to implement globally or even locally. There isn't currently a repository for such ideas.
I think that it would be useful to have a big list of feature requests and suggestions for some fantasy future MO 3.0, to serve as feedback data to MO and to SE, and to serve as an incubator for possible potential future feature suggestions. Having these voted up or down by users adds information, and comments on these are also useful.
Big list: Please suggest features, and make specific suggestions, for things you would want to see on a fantasy future MO 3.0. These don't have to be immediately implementable or likely to be implemented. One fantasy request per answer, please. Please try to avoid duplicate answers.
I'll start the ball rolling by inserting some sample answers. Please comment and vote up or down depending on whether these seem like good ideas to you, in the abstract.
I think the longer this list is, the more informative it is.