I'm sticking to realistic here, since we know a lot of the stuff people want just won't happen (Warlord class).
1) Backwards compatible, in the AD&D sense. Someone could be playing a character built using 5E, yet still play in the same game. Some mix & match might even be possible, such as 1D&D origins with 5E classes, but no 5E subclasses with 1D&D classes. Most books should still be largely usable, perhaps with an update (such as level requirements on feats).
2) Redo the fighter to be less generic. Battlemaster should be incorporated into the base class, while Champion goes the way of the dodo. Warlord returns as its own subclass, including non-magical healing (as a sub-class DMs can just ban it if they don't like it). Keep the Eldritch Knight and add archer and maybe knight as the other sub-classess to give a good variety of themes.
3) More DM help. The DMG is pretty terrible in teaching how to be a DM. There are rules for exploration and social encounters, but they're so scattered and disconnected as to be nearly useless. Better explanation on how to use secret doors, trick, and traps would be ideal.