WilderMount

A special enchanted saddle or lead that allows you to mount and ride (controlling motion/direction/speed) of nearly any passive mob(think Dolphins for speedy water travel, Strider for lava lake travel, iron golems) and select hostile mobs (like spiders allowing you to climb walls, slimes letting you jump and bounce high and negate fall damage, endermen letting you short tp like you used a pearl)

Use of the item could or could not have a cost. Such as needing an item in your inventory to consume that the mob likes. Such as fish for a Dolphin, warped fungi for a strider, poppy for a golem. 

I think this would be a lot of fun for players and open up more ways to play and experience the world.