Personally, I would enhance D&D monsters by deleting the Monster Manual − and instead, writing numerous Monster Manuals that are specific to each official setting. There would be a Greyhawk/Blackmoor bestiary, a Faerun bestiary, an Underdark bestiary, an Eberron bestiary, a Dark Sun bestiary, a Feywild bestiary, a Shadowfell bestiary, a Radiant (ethereal) bestiary, etcetera.
Each bestiary would double down on the flavor and ecology of how the creature fits into its setting. The approach encourages the DM to modify and customize each monster entry, to make it more suitable for the setting that the DM is running.
Also there is an SRD version of each creature that is minimalist and flavorless to help the DM customize its flavor.
Ultimately, there would be both lots of flavor to spark the imagination, and freedom to create new worlds.