So someone made this interesting mod that allows the Shield of the Old Ones to also block projectiles: https://www.reddit.com/r/totalwar/comments/9wnpxw/i_just_released_a_mod_that_turns_the_shield_of/
Honestly, this would make it far more useful then what it is now, as people only use it for the small leadership boon and occasionally against spells. And it would add a nice flavor to it!
Perhaps it the Shield of the Old Ones could additionally gain a -15 second cooldown reduction for each spell that is hampered by it's magic resistance? That way you will be rewarded for using it on top of your units. Though this last idea is optional.