I thought this might be helpful for the current leak season. There is an interesting history of WoW store mounts leaking themes and concepts for the next expansion BEFORE the next expansion is announced.
Here are the examples (it seemingly started around MoP);
MoP hint for WoD: Enchanted Fey Dragon: This mount was a preview for the massive Enchanted Fey Dragons of Shadowmoon.
WoD hint for Legion : Mystic Runesaber: This mount was a preview for the mount used by the Nightbourne.
Legion hint for BFA: Stormwind Skychaser/Orgrimmar Interceptor. Indicating Horde and Alliance conflict. (special thanks to @ElitePeon)
BFA hint for Shadowlands : Vulpine Familiar and Sylverian Dreamer: Both of these mounts hinted the Shadowlands and particularly Ardenweald.
Recent mounts:
Steamscale Incinerator (Mechanical dragon, released at the tail end of BFA after Shadowlands was announced, also part of Dragon Pack promotion last month)
Sapphire Skyblazer (Blue Phoenix. Seems more like a homage to TBC classic)
Sunwarmed Feline (Giant cat, bears some resemblance to red dragons and Alexstraza)
Of those, the Steamscale Incinerator seems like the most likely to be a hint. Not only is it a dragon, but another mechanical dragon (the Timeless Mechanical Dragonling pet) was released this past year. Sunwarmed Furline is another very likely hint.
Here is the lore behind the Steamscale;
Sounds very fitting for a Dragon-based expansion. However, I'd be interested in hearing comments from other posters.The fearsome Steamscale Incinerator is a spark-breathing robo-terror that rules the skies wherever it flies. Legends say a mad gnomish inventor had a deranged dream to ride a dragon, so he built this metal monstrosity powered with a trapped demon in order to create "an authentic dragon soul."
**Honorable Mention***
WotLK hint for MoP: Pandaren Monk Pet. Not a mount, but was also a store item. Released 2 expansions before MoP.