Link Belt to potions -> add new affixes/properties.

It's only normal that the potions are kept inside the belt.
Therefore my suggestion is 3 fold:

1. Make all chars start with a level 1, no property belt.
2. Make unequipping a belt remove all potion slots, i.e you cannot carry potions without having a belt equipped.
^ these upper two are just for flavor.

Now, the real suggestion:
Add a new type of affix/property to belts that imbue potions in the belt slot.

A few examples:
- "Potion in the 1st slot recovers 1 extra charge per rare monster kill"
- "All mana potions in this belt increase your maximum mana by x when used"
- "Reduced number of potion slots" - as a penalty
- "Cannot carry health potions" - another penalty
- "Activating the potion in the 2nd slot grants you 50% extra spell dmg for the duration"

et cetera.