I'd like to see a named set of Thieves's Tools that have a percentage chance to not disappear upon being used.
Maybe even add a tools slot to our inventory UI, in the same way that we have a Quiver slot, and allow us to equip sets of tools, rather than simply having them removed from our inventory.
The latter idea would free up some backpack space on rogues, which is a real problem since they typically have both TWF and UMD, resulting in a lot of pack space being used. This would also open some design space for different types of tools and some named variants.
Mostly, I want a set of 75% (or better) Returning +5 Thieves' Tools with a Green slot. I'm tired of filling my backpack and banks with stacks of +5 tools and then still finding myself without when I go to disable a trap, because I didn't realize that the last quest chain I did ate up 35 tools.