I don't like 4e dragons.
It's not because there aren't enough of them. I don't think we really need an entire menagerie.
It's not because they're not good in combat. They pose a threat in 4e, and they do it fine.
It's because they are unimaginative and simplistic in design.
The poster-boy for this is the blue dragon. Pre-4e, they were illusionists and deceivers, who wore a party journeying in the desert down over time, using their environment and their potent illusion skills to their advantage.
The 4e blue dragon shoots a lot of lightning. It is conceptually identical to a Dire Pikachu.
This is not the iconic villain that I want out of a dragon. Decent combat stats aren't good enough.
I need dragons that are complex, motivated villains. I need dragons with a variety of powers and abilities (not just elemental damage spam). I need dragons that threaten the party in all three pillars -- with difficult lairs, with vicious combat ability, and with dynamic personal presence. I need dragons the party will anticipate facing, and who will destroy a party who simply stumbles on them.
Dragons in 4e only become that if you put a LOT of extra effort into making them that way. I need dragons in 5e to be that right out the gate.
And, please, let's focus on getting the classic five right before you just drop more dragons into the game. White, Black, Green, Blue, Red (+ maybe Gold). Make these the definitive and iconic villains of the game, and I might be mostly happy.
This is a challenge. D&D, for all it has dragons in the title, hasn't done dragons up to my monstrous standards...really ever. 4e's dragons are waaaaay too simplistic, and tend to jettison their pre-4e flavor and abilities. Earlier dragons with their spellcasting are too complex, forcing me to make an entire spell list if I want to run one (not fun times).
When someone looks at the chromatic dragons of 5e, they should not see a random rainbow of catch-as, catch-can abilities. They should see distinct personalities, powers, and abilities. They should see entire power structures. They should see vast lairs, epic battles, and cunning villains. The reaction should not be, "Hur hur, let's add yellow and pink dragons," it should be "The White Dragons are assembling a barbarian army in the North! The Black Dragons are spreading plague in the South! The Blue Dragons are causing civilizations to disappear into the desert! The Green Dragons render the forests impassible! The Red Dragons are blowing everything up looking for their missing treasure!"
...or somesuch.
Bland dragons are unacceptable.