A. Setting: Home Brew///D&D3.5e
B. Basic Premise or Idea you wanted to experiment with.
I wanted to have magic suddenly appear in a world that had been at war for a long time. One kingdom decides magic is evil, the other decide that it's good. How does the world change? How does the war change?
C. Things you had to implement to make it work.
I gave the PC's partial gestalt: A gestalted caster level at 4/8/12/16/20 --- arcane casting only, no healing, conjuration. New magic is powerful, but it is still minor in comparison a standard D&D3.5E game
D. have you actually managed to run this campaign? Results?
Twice, both ongoing. The first one is PBP, and the players started at level 1, and are now close to level 6 (over nearly 4 years, admittedly slow). They were the ones to trigger magic returning to the world, and have since completed a few more plots.
The other is IRL. It started a few days after magic returned, with the players being rounded up for having magic. Since then, they've discovered the 3 main enemies (Negaters, mage chasers/slayers from the kingdom that hates magic, an invading force of barbarians from the northern sea, and the dopplegangers which are trying to throw the entire world into chaos for their own reasons).
One challenge I've had is providing magical rewards. At first, I provided slews of items that were impossible to identify. Eventually I started to allow identify spells to work slowly (requires 3 castings). One of the things that the group does is discovers and hunts down ancient magics for the mages power.
Should be an interesting campaign. It's neat how players interact with their relatively weak magics, and thinking about how a world responds; some people not trusting mages, some hating them, and some desperately wanting that power. There are other houserules in place, such as Augment classes (Taking "bard" levels at 1, 6, and 12 gets you bardic music with no other bard class abilities).