Allowing mounts to fight along side you would be great. Manacutter could very well be the Ark of FFXIV, transforming upon summoning for battle.

At the current moment mounts are just a thing that people mindlessly collect, allowing them to be levelable companions would give them purpose and personality.

There's already battle animations for quite a number of mounts including the Coeurl, Wyvern, Twintania, Behemoth, and more. Utilizing these animations that already exist would be a good feature for suffering through fates and such solo.