What if Elise's passive were made to...
Increase her Armor and/or Magic resist by X amount per spiderling on the field? That way its conditional and she has to pay mana to keep her tankiness and its only in her melee form. I think it would be really cool to have some bonus for having the maximum amount of spiderlings possible. Or maybe if she had the max spiderlings on the field she would gain X MR/Armor?