With over 500 mounts currently available in game - not counting cash shop and retired models, or even those from less reliable sources, such as MoP world bosses and Soundless - and Shadowlands set to bring a lot more in the near future, it's time for Blizzard to add some new mount collecting achievements. What we were offered at 400 was admittedly a bit of a disappointment, but Ion said that they will listen to player suggestions more this expansion, so I'll be hopeful that this applies to collectors as well and get the ball rolling early.
If we are looking at existing models, there is one obvious choice that should occupy one of these slots: the Brutosaur - both because this would be a nice consolation prize for collectors who won't be able to buy the 5,000,000 gold version, and because it is one of the fanciest models available in BFA, worthy of such an achievement. The Albino (Silver) Brutosaur would be particularly fitting, echoing the Albino Drake we got for the first ever mount collecting achievement:
Ideally, if they really want to show us some love, they will make two separate versions of the mount: one with a Zandalari/Vulpera themed saddle for the Horde and one with an Explorer's League themed saddle for the Alliance (because the Alliance should be able to ride dinosaurs without feeling like wannabe trolls!). It's been a while since the Horde and Alliance got separate mounts from the collectors' achievements and I kind of miss that.
An alternative to the Albino (although I'm partial to it because of the drake connection and the implied rarity) would be to give the Horde and the Alliance different colored Brutosaurs:
A Brown one for the Horde because these are the ones we see walking around in Zandalar the most and the coloring matches the existing Troll-themed saddle very well:
And a Dark one for the Alliance (preferably with a new or recolored saddle), because the black and purple go well with a lot of Alliance races (Night Elves, Draenei, Worgen, Void Elves, Dark Irons:
It would also be awesome if these Brutosaurs come with their own utility NPCs, specifically a vendor and a Black Market auctioneer (echoing the standard auctioneer on the gold sink version). We can already get a consumable item that grants remote access to the BMAH from a Mechagon daily, so this wouldn't be game breaking, and it would be a neat little functionality for collectors to have.
For the other slot, there are few options I can think of when looking at the remaining unimplemented models. While there are plenty of mounts there that I will like to see added to the game, most of them would fare better as rare drops or rewards for various other activities. A mount collecting reward should stand out.
There is the cool-looking chimera from the old WoD files, but since the Darkshore mount established that Azerothian chimeras look just like Rylaks, we don't know where this creature should be coming from or it even exists in the lore:
There are the Legion Rocs, which obviously could be mounts because they use the same skeleton as the dread ravens, but they were never actually implemented as such. Slapping a saddle on them would make them appealing mounts, and they even come in Horde and Alliance colors:
Also cool but requiring saddles are the Draenor Hydras (I never understood why they weren't added as mounts in that expansion), but they don't really fit the theme of either Shadowlands or BFA. Still, I do hope we'll be able to ride them some day:
Finally, it would be nice and refreshing to see a cool looking new model added in Shadowlands specifically for us. Not necessarily something flashy and bright that screams "store mount", but something that says "rare and unusual specimen" or "collector". Or a fancy multi-passenger vehicle (4 party members riding inside?), such as a Vulpera caravan wagon for Horde and an updated Gilnean stagecoach for the Alliance (it mostly needs to use the new horse models and maybe a few retouches):