One of my biggest gameplay annoyances has always been the line of site limitations for casting. I would suggest a line of enhancements and/or feats that augmented a casters ability to avoid spell failures based on line of site issues. For example...
- Casting around corners : allowing a limited ability to cast spells at a curve, avoiding some obstructions.
- Arms length casting : Allow the casting of touch range spells in any direction, regardless of line of sight
- Peripheral casting : Increasing the "sight-cone" for the purposes of casting from the ~ 90 degrees it starts up to 180 degree (maybe in stages.)
Frankly, there are a lot of builds designed to get into the fray, and this sort of thing could be useful to a lot of builds and would, in no way, be overpowering.