Blocking magic
When it comes to the really uber big spells like volcano, it would be unbelievably frustrating if your prized capital was destroyed by a sneaky opponent when you are steamrollering elsewhere. Are there going to be methods of dispersing magic? I mean there's not much point in big armies if they're just going to get shoved a volcano you know where.
Perhaps sovreigns (and maybe imbued heroes) could have the ability to prevent the really big spells wihin a small radius of themselves. Minor things like heal and stuff are too small to be stopped, but you're not going to be hitting volcano under Mr Sovreign. Otherwise I can imagine late game battles never taking place due to one army or the other using the magical equivalent of nuking.