Moving things to and from the main inventory to the saddlebag to free up space can be a pain when I'm looking for stackable items that are in Inventory, but also in my Saddlebag. A "Consolidate To Saddlebag" command in the Inventory popup menu could be helpful. It should find all stackable items that are in both the Inventory and Saddlebag and move/stack the items, after confirmation, to the Saddlebag.

Perhaps it would be useful for this to be more general, and work in either direction (inventory->saddlebag and saddlebag->inventory), and maybe with other alternate inventories (like retainer, and maybe others that I'm not aware of yet because I'm still kinda new).