Default MIN/MAX? No thanks.

This doesn't increase diversity, it decreases diversity. If every option scales to whatever is highest, this leaves a few correct choices and a whole bunch of wrong choices for everyone.

Less restriction creates less diversity, as counter intuitive as that might sound. A game with 0 restriction will result in a singular optimal build despite the number of options available. As restriction increases, trade offs are made, minimising weaknesses and maximising strengths, and the meta game forms around a larger number of options. A game with 1 restriction will have at least 2 optimal builds, with the number of optimal builds increasing with the number of restrictions.

Extremely large amounts of restriction, say if each character could only use a single skill, also results in the effect of less diversity as well. I.E. if a heal is more valuable than any other skill, and you can only use one skill, everyone will choose that option. Finding the median point where restriction is neither too high nor too low to permit diversity is the key. Making some stamina morphs has decreased the restriction a bit for stamina builds and created more diversity, but if they make every skill this way, that diversity will vanish, and there will be only one optimal stamina configuration for every class. Though I'd argue it would lose all of its appeal since Magicka still doesn't have an opportunity cost (CC breaking, Dodging, Sprinting, Sneaking, Blocking, Bashing, etc.).

Some things should scale on weapon power, some things should scale on spell power, and focusing on one should come at the cost of the other.