As I already wrote here, the issue lies in the mechanics themselves. There's a stacking mechanic. How do you justify being able to stack items everywhere, but not in your main inventory?
You won't compensate for the ability to stack items in your main backpack with new bags or upgrades. You'll reduce your main inventory by 3-5 times. Therefore, point 1 is only possible if you offer an infinite inventory in exchange, as gravesmerch wrote.
gravesmerch wrote:best I can offer is Salem's infinite scrolling inventory
But points 2 and 3 would be welcomed.
My english is bEd.