I really hope that finally after 3 years since BFA pre patch July 17 2018, we’ll see a new mount achievements for 500+ mounts.
There’s a point regarding new toys, Mounts and Travel Forms,
Please bring something new to the mount collectors.
As mentioned by Melaeni on comments, Also improve Mount Tab with Favorites per Character, that will do some things like mount list that will match your transmog for each character or your roleplay…etc
At the very least 450 and 500 respectively. And no reuse of old models. Give us something new to strive for!
I would really love to see a carriage mount like the ones in Revendreth. 
It’s baffling to me that 400 is still currently the most recent achievement. I’m currently at 571, and 600+ seems completely plausible at this point.
Mount Collectors doesn’t have goals, usually a new achievement is added and we already have that goal completed. We don’t have insight on how they decide to include these achievements since sometimes they take 3+ years to add more with recolors.
Last I heard, they didn’t want to put any new achievements in for it due to how intimidating it would seem to newer players. Kinda lame but there you go 
While they’re at it, can we also have: Favorite Mounts per character instead of account wide and Sort Mounts based on Flying and Ground Only.
I use a macro and the Litemount add-on for that. I shouldn’t have to but…there you go. Either way, it’s a far superior system to how the default client does it.
This
I mean i have a macro for it but still. Should be in game
I used to want a 500-mount achievement, but with the flooding of mounts into the game with this expansion, such an achievement will feel near-worthless.
That said, the achievement should still exist for those who want it.
I want the mount to literally be the word Mount and you just ride around on it
I’ve also heard that. But i don’t think it makes much sense. Cause if you look at the 500 honor mount, a lot of players are super far from that. And there are dozens of other achievs in-game that requires tons and tons of time to complete them.