I think it would be great if you could add infinite upgrades for stack sizes and weapons and armour for the new game plus levels like it is with the health and stamina upgrades

Yeah that would be cool, like maybe they could add a way to upgrade your backpack and instead of weapons capping at level 15 they can still be upgraded it's just every level would need 5 more fusion crystals than the last level to upgrade (like how it is when leveling up a weapon from 9-15) or maybe it could be a little easier to upgrade weapons than that. Then for armor they could do something similar to weapons, but the upgrade item for armor could be different than fusion crystals or something like that. Because being able to upgrade armor past level 9 would be nice.