A few things I have thought of that I think would be awesome .. either a stable where we can store just mounts or a Kennel to store just pets or maybe both .. On the subject of Pets how about .. Morphing shadow pets .. a May cast spell like with the weapons .. only available after you get shadow magic
I must admit, I am confused as to your exact question, but feel that the overall point is rather mute. You can currently store up to WELL over 100 items in a Castle at any given time so placing down mounts/pets should be a rather minuscule task. In addition, you also have the Bank, Shared Bank, Attic and other castles in order to store all your collected items, pets and trinkets as you see fit.