Suggestion- Add different backpacks you can upgrade into. Upgraded backpacks start small again, then you have to resize them up again. Maybe after beating special bosses you can choose one. There could be various packs, each with their own quirk, like weird layouts, segmented sections, "holes" to pack around. The upsides could be things like powerful special slot multipliers, specialized build modifiers like half mana cost, gold for every kill, double potion effects, large max size, etc.
This could build upon the depth of the game mechanics, while offering more progression, and keeping the challenge of fitting things in a bag.