I'm happy about an additional mog backpack/basket/tupperware container. Porter service in MH is nice too.

I'm throwing my vote to more space in each bag though. Having to slim down optimal sets is heartbreaking. In lieu of more physical slots, I'd actually prefer Kari's suggestion of items with combinations of stats and effects. For example, a quest to combine all obis or magian damage staves once you've obtained all 8.