Make the carrying capacity based on weight rather than stack size. It's really painful having to spend ages moving full inventories of 999 items, please increase the max stack size to 9999.
In-game Name: Mat
I'm not excited by making the stack size for all items 9999, but I think SBI should seriously consider doing it for extremely cheap and plentiful ones.
My real target here would be materials such as runes. Stuff that just seems to pile up quickly and takes up excessive amounts of storage while generally not being all that valuable.