Spell DC

During a recent session it became apparent to me that I forgot to include the difficulty ramp for spell casting.  The rules say spell casting is DC 10 and the chance to successfully cast a spell is:

 Roll + Level + MIND bonus >= DC 

Thus a reasonably intelligent caster with a +2 MIND bonus would fail on a 7 at level 1 by level 7 would only fail with a 1 (the D20 5%).  Well this is fine but not really too dramatic and it does not add any provision for the spell level.  I had forgotten my original intent - that the DC would be 10 + spell level.  I graphed this out to see what it would look like for both lowest and highest spells per level:

DC 10 +1 per spell level

Well this is better than maxing out the whole system at 7th level, but clearly the caster level bonus totally overtakes the spell difficulty increase.  There are some options to deal with this so that some tension is kept in the system.  One way would be to increase the DC modifier for spell level or decrease the caster level bonus.  These approaches simply cancel out and create extra math and I don't want to do it though because I really I like the simplicity of +1 per level, however if I were to do it I would change the caster bonus. Folks are used to attack bonus principles and gaining ability when they level. That would look like this:

Caster bonus every second level.

...and that looks like it really punishes casters.  It basically freezes them at a 50% chance for their highest level spells.  Lowest level spells suffer as well - having cantrips fail 10% of the time for a bright level 11 spell caster is not right.  No, not liking this so it looks like we'll stick with the first table.

But that sharp curve needs to be addressed somehow so that the whole mechanic does not become irrelevant, what are some other options?  Well DC is all about Difficulty and what makes spells more difficult to cast?  Complications!  Apparently, the system works after all! First there are the standard complications of changing the spell itself - Extending, Empowering and Widening should all include a DC modifier.  Then there are DC modifiers for environmental conditions - I imagine casting a spell from horseback or in a hurricane (or when falling, or in deep water...) would be more difficult than from a quiet neutral stance. Actually this is good because it rewards the high level caster, letting them become more proficient and allows the GM to adjudicate DC as it was intended - as a difficulty modifier.  I like it - a high level caster can attempt things a low level caster wouldn't - like a fireball while rollerskating.  Also they might be able to cast cantrips while caught in a tornado but just might have some difficulty with that 7th level spell.