Blizzard notoriously don't want to put truly unique mount rewards behind the mount collection achievements because it's a very daunting task for any newcomer or people who have played for a long time but never cared to collect mounts.
I can't even imagine having to begin from scratch on collecting the ~550 mounts I'm currently at. But the last time we had a truly unique mount reward from an achievement was the armored dragonhawk at 200 mounts back in MoP.
Since the armored dragonhawk we've only ever had unused recolours of mounts added as rewards through the collection achievements.
And with the 350 achievement rewarding an infernal, it's not unlikely that any additional achievements might reward a non-flying mount like a recoloured brutosaur.
That said, at this point I feel like it's unlikely we'll even see a new mount collection achievement during SL. It really doesn't seem like much of a priority for Blizzard in the first place, and currently even less so as they struggle to get new content out.