You can't let elemental type be selectable. Everybody will choose their highest percent increase. If I have 25% increase fire damage, versus a max of 20% for any other element, I will always choose fire. It does more damage.
This is pretty much what OP was asking. Builds defined by their function, not their output. Personally, I love the way D3 is headed with builds and skills and stuff. I just wish updates would come faster.There is plenty of diversity in the game, other builds will always prevail in this kind of game, like they do all games.
Personally I think it's rather bland if runes are so... I don't know, bland, cosmetic, and just don't hold any real significance in the building blocks of a refined build.
Some builds are for pushing (inna gen, inna EP) some are for speed grifts, some are for tx farming/bounties.
If the developers want to make drastic changes they will [A] either come out with another expansion or Diablo 4.
If you're interested I made a post about diversity here. http://www.diablofans.com/forums/diablo-iii-general-forums/diablo-iii-general-discussion/161238-illusion-of-diversity