Here is a concept I came up with:
What are mounts?
Mounts are animals that your champions can ride into battle, providing minor stats to the champion, and a small boost to the party.
How do I get mounts?
Mounts are crafted, using a new worker who is an animal breeder.
Where do I keep my mounts? How many mounts can I have?
Mounts are stored in a stable, which is a new part of the shop. It can be expanded, for gold or gems, to store more mounts. Like other shop expansions, exapanding the stable with gold requires a certain merchant level.
How do I use my mount?
When you select a champion in a quest party, you can add a mount to your party if any are available.
What kind of bonuses do I get from my mounts?
Different types of mounts provide different bonuses.
The stats (e.g. ATK, DEF, HP) that a mount provides to a champion are based on the quality and tier of the mount.
The party bonus provided is based on the heroes affinity for riding that type of mount.
Examples:
Donkey - This miniature beast of burden can carry anything, providing you with an additional 5%/10%/15%/20%/25% components.
Giant Tortoise - The tortoise provides your party with an additional 2%/4%/6%/8%/10% defense.
Unicorn - The majesty of the unicorn motivates your champions, providing a scaling bonus to their inate skills.
Dragon - The dragon flies your party into battle, reducing quest time by 5%/10%/15%/20%/25%.
What is mount affinity?
Your champions are unfamiliar with riding animals, and so they must learn. As a champions rides the same type of mount into battle more, they gain an affinity for that type of mount. At various affinity milestones, the party bonus levels up.
What else should I know about mounts?
Your mounts get tired. They need to rest before they can go into battle again. This can be reduced by feeding them meals or desserts.
Your mounts need to be broken in. When you add a mount to your stable, there is a delay before they are able to go on their first quest.