Pretty interesting concept. "Thematic" House Templates which hold more of it's correlated item. I think rather then making a specific dedicated house for that it could be a "Template" you purchase to upgrade any house.
For example: You buy the Template, apply it to a house of your choosing. That House now can hold More Pets/Mounts.
"You've applied the Ranch Template to your Amaya Lake Lodge, it can now hold more Pets/Mounts!"
Lots of potential Templates as well.
You're idea of a Pet/Mount one.
"Library" capable of vastly more books/scrolls
"Museum" with relics/legendary furnishings etc
"Restaurant" for food items.
"Conservatory" for Plant/Garden Items
Ideally in a perfect world, we just see the much disdained Item Limit get improved overall and be done with it. But maybe a more organized, controlled approach to increasing decorating potential would be "easier" for Zenimax?