ok what would you think about instead of just a general mana pool you also had different types of elemental mana as well?  basically you would have water, earth, fire, and air mana alongside general mana.  the idea is this.  general mana is used for casting all spells.  its your main power reserve.  then when you get a shard you also start building up a store of elemental mana crystals.

ok now these can be used in a variety of ways.  you would have 4 power scores.  one for each of the elements.  you would then use these crystals to increase that score.  the higher that score the more powerful spells from that category would be.

you could also use these crystals to make spells instantly more powerful.  it would be a great sacrifice to use them like that, but it would be a kind of emergency use thing.  you would use fire crystals to make fire spell more powerful and vice versa.  keep in mind that all spells are cast from regular mana.  the crystals would just augment the spells effect.

you could use them in crafting magic items.  for that sword of burning wind you may need 50 fire crystals and 50 air crystals and 100 mana to cast.

you could use them in diplomacy as powerful currency.

so what do you think?  maybe for an expansion?