The best mounts are currently in Guild Wars 2. They had a very interesting approach to the mount system.
Instead of having a standard mount system that you just spawn your mount, hop on it and go, they introduced different types.
Each mount, be that glider, wolf, drake or roller beetle, had a very specific gameplay that is unique for that respective mount.
Also you are not dismounted the moment you are hit by a spell/attack, or your mount instantly disappearing after touching water (ESO).
I found that amazing and nothing we had seen before.
I say think about what mounts will introduce in the game:
1. Are they going to be tied up to the story?
2. Are they going to serve a different purpose then a mere transportation?
3. Would you be able to change their color, equipment, upgrades perhaps?
4. Can you transport goods or anything else if need be?
5. Will they offer something different or unique like let's say underground traversal?
6. Will be like a virtual pet that will die of starvation or thirst?
7. Are there going to be bonuses to your pet if you keep feeding it, stats perhaps?
There are many possibilities.
I do like the direction you are going though. Making us strive for whatever feels engaging and going back to the core of MMOs.
If I have to be serious I prefer the old style of gameplay where it's a struggle to complete the content.
Not too much, not too little. Just the right amount so you can feel that you actually completed something.
For example Classic WoW, or Guild Wars 1.
I really miss the opportunity in MMOs instead of having millions of different animals to ride, to have a few but to be meaningful.
For example 10 different mounts but you struggle through content and quest chains to acquire them.
Each feels like an achievement, you feel connected with your mount and you can upgrade it, pet it, feed it and get customization for.
In fact you can tie quest progression or item searching (much like archaeology) with mounts. They can have their own abilities.
Just food for thought I guess, nothing concrete....