This idea literally popped into my head fully-formed as I was waking up this morning. I've been thinking on and off about my simple-minded initiative rules, whereby there's never really a chance that a caster will get their spell disrupted. Under my present refereeing spellcasting is pretty much an instantaneous affair. I don't do a round of declaring intentions before initiative, so no one ever finds themselves in a situation where they are casting a spell when the baddies win inish and wreck their plans.
Your spell getting blown is a risk that I want magic-users to have to consider without messing up my super-simple initiative rules and I think a back corner of my brain was working on it will I was asleep last night. Fully conscious Jeff wasn't smart enough to look at MERP's spellcasting system for inspiration, but I eventually got it.
Under this proposal, spellcasters can continue to use instantaneous magic if they are willing to accept a risk of fumble equal to the spell level or less on d20. E.g. a third level spell thrown instantaneously fumbles on a 1, 2, or 3 on 1d20. Power Word spells would count as 1st level for these purposes, as would anything on cast from a scroll.
As an alternative, the caster can state "I begin casting [spell]". The following round the caster can throw the spell at -1 to the fumble chances, state "I continue to cast [same spell]", or state an all new action (i.e. canceling a spell is penalty-free). If the caster is hurt between beginning the spell and actually casting it, the spell is lost. Each additional round of casting lowers the fumble chance. As I woke up it was an additional -1 per round, but that really slows down casting higher level spells safely. Now that I'm thinking slightly rationally, perhaps each round should double the reduction of fumble chances: -2 for 2 rounds prep, -4 for 3 rounds, -8 for 4 rounds prep, etc.
Since I just woke up a wee bit ago I don't have that fumble chart yet, but obviously some bad stuff can happen. However, there would be a few escape hatches built into the chart: results that would allow the fumble to be avoided or diminished with a successful Int or Dex roll, or maybe a level check or something like that. I like systems with built-in narrow escapes like that.
Random Hexless Terrain Tables
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