Mine's one that will probably catch some flak around these parts. I hate how Evasion and its ilk allow you to dodge between the heat molecules of an incoming firestorm.
If I were going to patch it for realism's sake, I'd let anyone with Evasion move up to their base speed on a successful save. If it put them out of the AoE, they'd take no damage. Once they're out of the AoE, they're movement ends.
Doesn't solve the issue that happens by RAW when a 3rd level Rogue is in a 2.5' x 2.5' broom closet when he triggers a Fireball trap, and somehow takes no damage.
If they can't get out of the AoE, then they still take half damage as if they didn't have Evasion.
Although, I honestly wouldn't bother with the change, personally. I just threw it as an idea out there to get them out of the AoE all together, so you don't have to worry about them dodging within the AoE.