I've encountered three types of inventory.
Weight based inventory
Space/shape based inventory
And the slot inventory.

On occasion I will see a hybridized version (Valheim of course uses weight and slots). I understand the desire for a backpack and sometimes things about slot inventory may seem silly, but I see it as more of a puzzle. Each different inventory system presents different challenges.

Weight based is probably the most simple.
Shape/Space is usually the most complicated/realistic.
Slots is a nice middle ground in my opinion.