Age of Empires II: Definitive Edition has recently been rated by ESRB, which means that - inevitably - we will be getting a full remake of the legendary AOE II sometime in the (relatively) near future!
Do keep in mind that the fact that the first three Age of Empires games are getting full remakes isn't exactly new, but thanks to ESRB, now we know that we're not all that far from having a modernised AOE 2 in our paws.
In accordance to ESRB, Age of Empires II: Definitive Edition has been rated for teens, and will expose players to hearing the word 'bastard', as well as to seeing a few bloodstains on the ground. This sort of stuff aside, AOE II: Definitive Edition is definitely going to feature vastly improved graphics, as well as a modernised UI, much like its predecessor did.
There's also been word of the game coming pre-packed with updated versions of all of the original release's expansion packs (Conquerors, Age of Kings, Forgotten Empires), though there's been no official confirmation on that.
If you're confused on how Age of Empires II: HD differs from Age of Empires II: Definitive Edition, Definitive Edition delivers a fully reworked and updated version of the game, while the HD version simply upscales textures and modernises the backend of the game. If you're in the market for a bit of nostalgia, but also wouldn't want to give up on two decades of game development progress, be on the lookout for Definitive Edition info.