While I think it would be cool to play a Venthyr, I'm pretty clueless on what blizz is planning. However, I think I know one way they could be made to fit the story.
There are hints that the next big threat will be the void and/or the light, and the light has already begun encroaching on the Shadowlands. The Covenants joining us in the struggle against void & light seems plausible, tbh. But an allied race has to have a reason to come to Azeroth, I hear you cry! Not necessarily. It's unknown how the levelling system will work after SL, but one option is that we reset to 50 at every expac launch. Fresh characters now canonically go to Exile's Reach and then to Kul Tiras and Zandalar. Next expac would then either set the Shadowlands as the new default, or retain BfA zones while adding SL to chromie time.
Enter covenant allied races: they start at level 10 like all other SL races, and default to SL content even if BfA remains the default. When they reach 50, they go to the new zones like everybody else, which is in the thick of the fight against the void (and light). Covenant races *can* enter Azeroth and even level there, but canonically they don't. If they come to Orgrimmar or Stormwind it's as just that, allied troops in our fight against the void (and light).
And the expac after that they're legacy content and are mentioned about as often as the Vulpera or Draenei are.
(Yes, joining covenants as that race will be silly. Yes, you can already see that by levelling a Kul Tiran in Kul Tiras, they still call you "mainlander" constantly.)
edit: This can be "the thing" next prepatch. When we're all done with SL and are just waiting for the next one, they launch four new allied races and reset us to level 50, letting us again level new alts for the next expansion.