Unstable Mana: magic system variant.

[Giovanni]

Alternately, have no "mana pool" do a straight up "gathering energy" as part of a spell's casting. In this setup when they cast a spell they roll the spell's Skill, the ritual length is the Casting Time (yes so spell skill does double duty here - but this means at low skill level spells take much longer so that's something to consider, it makes less skilled mages vastly 'weaker' than high skilled mages), at the end of each Casting Time they roll spell skill and 'gather' the MoS of the roll in Mana Dice. Those dice are immediately rolled to generate mana for the spell. All Mana Dice rolled generate 1 (or more) mana, 1 can be held aside in a "mana pool" and not used right now or they may be spent to generate 1 mana and be immediately rerolled for the possibility of extra mana right now, 2-5 are expended immediately, 6s are expended immediately but also go "wild" as the spell is cast. Once enough Mana is generated, the spell is cast (no further rolls are needed) and any 6s rolled are used on the Wild Spell Chart.

Sure: I prefer this variant.
It looks very good.
Maybe 1+MoS is the maximum mana dice you can gather.
So, for a low cost spell, if you want to go with a sure cast you can.
But very expensive spells will require both high skill and high risk.

EDIT:
1 on a mana dice could be
* 1 point of the spell cost covered and.....
* mana dice is rerolled.