Two of my players just asked me about an idea they had. One is playing a paladin, the other a druid (with a barbarian dip for the rage bonuses).
Their idea was that the paladin player would take the Mounted Combatant feat and the druid wildshape into a large animal that can carry him. Which would mean that when the paladin mounts the druid, the paladin whould for one get advantage on attack rolls against anything smaller than the druid's wild shape, the paladin could redirect any attacks aimed at the druid to himself and the druid would essentially have evasion.
I looked over the rules for mounted combat and the feat's description and saw no problems with the RAW. But I felt that, aside from the advantage on attacks the rider gets, the feat exists to allow a level 10+ character to ride into battle on a regular warhorse without the horse just being pounded into paste by a single attack from a CR appropriate enemy. So I decided to rule that you can only apply the effects protecting the mount if you are controlling the mount (as per the mounted combat rules, where a mount can act independently on its own initiative or be controlled by the rider and act according to the rider's inputs on the riders initiative).
I thought it made sense this way because it's the rider's feat and it doesn't magically makes the mount better at avoiding damage, but the rider is guiding the mount while controlling it.
Since the druid is an intelligent creature, the paladin would not be able to control it like a horse and not be able to apply the mounted combatant benefits, except for gaining advantage on attacks. I felt this was appropriate, using the existing rules and more in spirit with the feat's intention and it doesn't turn the two of them into a symbiotic tank.