Hi, I was thinking about a mod that takes the Morrowind way to cast spell. I think that it's not realistic that you can cast a spell only when your level is up to 25, 50 or 75. The percentage of chance to cast a spell depending to your level is more realistic. (the best caster of the world can fail too, and the crumyer can success!).

If someone have some ideas for the script, and can help me for doing this mod,

There is no way to change this. It is part of the hard coded game. Any sort of scripting would either take place after the spell was cast (and hit its target), or would be unable to determine what the player was aiming at (OBSE). And ti would require specific scripting for all spells, and would still probably not work on anything added by mods or made within the game.