I'm working on designing a new campaign world and one of the primary aspects I want to use is to capitalize on the elemental oppositions that are a trope of many RPG's. The world will be divided up into their elemental zones, which shape both the landscape and the power of elemental magic within them.
The idea would be to have the spells of the associated element be stronger in their own zone, but also be weakened in that of their opposition. I'm not just not sure how to do this mechanically without throwing off the balance of the game entirely.
I've considered a caster level bonus equal to half their level or maybe a flat die increase based on where they are in the zone (this would involve designing rings of power influence into the world as well). Theoretically, the penalties would be the reverse of the same system.
Any ideas or suggestions?
Particular spells and spell-like abilities are easier to use or more powerful in effect on planes with this trait than they are on the Material Plane. Natives of a plane with the enhanced magic trait are aware of which spells and spell-like abilities are enhanced, but planar travelers may have to discover this on their own. If a spell is enhanced, it functions as if its caster level was 2 higher than normal.
Particular spells and spell-like abilities are more difficult to cast on planes with this trait, often because the nature of the plane interferes with the spell. To cast an impeded spell, the caster must make a concentration check (DC 20 + the level of the spell). If the check fails, the spell does not function but is still lost as a prepared spell or spell slot. If the check succeeds, the spell functions normally.