I am playing a caster and, looking over the teamwork feats, I didn't find any that really jumped out at me, so I home-baked this one:
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Coordinated Casting (Teamwork Feat)
You have learned how to better coordinate the casting of your area-effect spells with your teammates.
Benefit: When you cast a spell that affects an area, any teammates in the area of effect that also have this feat get a +4 bonus to any saving throws against that spell. Additionally, if the saving throw is successful and would reduce damage by half, those teammates suffer no damage instead.
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I'm looking for some input from long-time players as to how balanced it seems. My GM thinks the evasion-like aspect is too much and that the save bonus alone would be just right. I pointed out a couple of things:
1) It's not full-blown Evasion - it only applies to your spells and both you and your ally must both have the feat.
2) When you compare, say, the Shielded Caster teamwork feat to the Combat Casting regular feat, Shielded Caster provides far greater benefits. The teamwork feat provides a bonus to all concentration checks whereas the regular feat only applies to defensive casting and concentration checks while grappled. Additionally, the teamwork feat can provide up to a +6 bonus (vs. the regular feat's +4) AND stacks with the regular feat AND halves the penalty you take if your opponent has the Disruptive feat.