Imo this is an impossibility, or at least so unlikely as to be effectively impossible. It'd be cool, sure, but I am highly skeptical that Larian would be able to implement and add whole new set of mechanics and models for mounts given all the other things they still need to do. They haven't even implemented some of the basic classes and spells yet...
- your allowed to use any mount that is the size same as yours. Unless heavy armour [then it needs to be +1 size]
- your mount size should increase, depending on class, every 2-4 levels.
Assuming "mount that is the same size as yours" means that your mount can be the same size as *you,* .....uhhhh no. There is no way a human is riding a wolf or a dog into battle. Mounts should be required to be 1 size larger than you, and possibly even larger (or have a certain minimum Str score) if you're wearing heavy armor.
- Or, are you trying to say that you can use any animal of a certain size - if your dedicated mount is a Large horse, you can use any other Large creature as a mount? But you can't mount a Huge creature? If so, that's fine.
Your mount increasing in size every 2-4 levels means that it'd be gargantuan size very quickly, which would be way too unwieldy and unrealistic. Why do mounts need to increase in size at all? And how?
Also, you run into the problem of when you're mounted on a Huge+ mount, it's unclear what areas you can actually attack or be attacked from. E.g., a human riding an elephant can't reasonably hit anything that's next to the elephant on the ground. At the very least, this size increase shouldn't be mandatory so that melee characters can continue to use their mounts.
- mounts could scale with Wisdom[for mounts that use magic] Or scale with strength / dexterity
The mount would scale to player -4 for mounts that are controlled by dominate beast. This would allow you to mount a beast, and attack it with party for an easy kill. It also opens a lot of ways for beat some hard encounters.
Regular mounts would scale to the level of the pc. Any players who take said classes as multi class would get the -4 scaling.
These seem contradictory..? Does the mount scale with the player's ability score (Wis/Str/Dex) or does it level up with the player?
The latter seems good: you'd at very least want the mount's HP to increase with your level. Its proficiency bonus could also increase.\
All this said, there are already rules in D&D 5e for
mounts and mounted combat, even a
feat. I don't think these rules are perfect, but it'd still make sense to start there and make modifications to those rules.