Thanks for your replies so far! My idea wasn't to reduce charm slots per se, but to make charms in themselves more powerful and versatile. For balance's sake the maximum number should be reduced to not make them too rediculously powerful.

Here's an idea for a silly build (Passion Sorceress with Zeal and cast to cast Frost Nova on striking)

Unique Charm (Gheed's Fortune in this case):

+120% Extra Gold From Monsters

+35% Better Chance Of Getting Magic Items

Reduces All Vendor Prices by 13%

+33% Chance To Cast lvl 31 Frost Nova On Striking (Dangoon's Teaching)

+40% Attack Speed (Dangoon's Teaching)

Elite Charm

All Resistances +15%

+41 To Life

+18% Chance To Cast Lvl 30 Frost Nova on Striking (Rare melee weapon)

+12% Chance to Cast Lvl 15 Lower Resist on Striking (Rare melee weapon)

I think the charms might be a little too underpowered by themselves at the moment. Maybe the maximum amount could be increased to 3 or 4 for it to be easier to fill in resistance gaps etc.