It would be fun if there were bosses that could be summoned by a ritual that must have will. First, it must take a lot of LP, second, the player must be present and forced into an arena fight, third, the type of will used will change the bosses mechanics, and fourth, a good drop for completing the fight. This would add to the end game and make it fun.

So Gaia Guardian but it takes LP

A little, but the Gaia Guardian has only one type of fight. Ideally, different wills will change the bosses AI, health, damage, armor and even drops, also, only one will type should be used to prevent massive amounts of code. For instance, raw will may make a base fight, where as corrosive will cause armor to degrade and cause poison or wither. Also, the Gaia Guardian is designed to not be farmable or one shot. Many of the similarities in the idea are in order to make it balanced.

PLEASE change the title.
And then add a suggestion label, I guess.