Savage Wombat
Hero
I don't know if I necessarily want to have it included, but I did very much like 4e's idea of the giants as Nephalim-like descendants of the elemental creators of the world, instead of just another humanoid race.
...and while there are select things from the 4e planes that could be poached for 5e...
I would like to read about dragons, rided dragons, dragon highlords, dragon orbs, and... yes you understood... dragonlances!
Dragonlance setting apart, I would like to read more about knights, peasants, castles and medieval lore. I think that D&D has got to be too much fantastic in last years. And too much dark: I prefer "regions of light" to "points of light".
More shining armors, white horses, sunny places on art please!
The Great Wheel and its three editions worth of development on the history and origins of the various D&D outsider races and their respective planes. A vast amount of that was gutted or radically transformed by the PoL cosmology in 4e, and while there are select things from the 4e planes that could be poached for 5e, I truly don't want to see the PoL planes imposed for a second edition in a row.