What about this?
No backpack: 2 rows plus hotbar
Tier 1: current inv minus 1 row
Tier 2: same as the current one
Tier 3: current inv plus 1 row + 1 slot hotbar
Tier 1: Basic item. Could be crafted without BPs and WBs. It would require cloth, leather and bones.
Tier 2: WB level 1 or level 2, or you could buy it at outpost for 100 scrap or you could find it in low level crates. It would require cloth, leather, rope and metal frags (or maybe tarp instead of cloth).
Tier 3: You could find it in high level crates. Cannot be crafted, but you are guaranteed to get one from a rig or cargo run.
This is my own idea, I would enjoy this being implemented in the game. And remember guys 10-20 slots would be too OP. It would ruin the whole game.