Fates should ultimately be a way to force the beast tribes to summon their primal....
The reward could just be existing beast tribe mounts but with an aura; Ifrit's would be the Firedrake with a fire aura/glow around it. Bismark's would be Sanuwa with wind + water aura on it
And since these rewards would have a timer on them that expires it'll have you coming back to fates to get it again in the future if you really want to use that mount again. And since it's existing mounts no one can complain.
I just want scenes like this that would really add dynamism to the game in real time
Ultimately this is that kind of recyclable content that would add so many points to the open world: lore (beast tribes get scared that you're doing so many fates in their habitat), easy reward with a cool touch (it could even be existing primal weapons with a bigger/different glow), etc.
The requirement would be something like completing 100 fates in the zone that primal appears.