One of Horizon's more undercooked mechanics is the ability to mount a small, select number of machines, and while a simply greater variety of mountable machines would be very much welcome, what a lot of fans are really craving is flying mounts.
Horizon 2 should allow Aloy to mount a Stormbird, for instance, to traverse the epic open world, effectively acting as a fast travel system of sorts. Naturally this also invites the prospect of aerial battles against enemy Stormbirds, and if it became a significant gameplay system in the sequel, it would be a fantastic way to one-up the brilliance of the original.
Just imagine being able to raid an enemy camp by flying in on a Stormbird, swooping down, grabbing an enemy, flying back into the air and dropping him from a great height. Sick, but awesome, right?