This is a good post, karma+ SG.
I've noticed it too, a lot of people are simply just adding 'anti-rainbow' or 'ideas for mono-gravity' etc.
On top of understanding the basic principle of effective deckbuilding (Diversity vs Efficiency) - it will help to take a step back and notice the simple cards in the game which have really powerful abilities. The first step of this is to realize that Creatures with Abilities are powerful.
You are limited to 6 cards in a deck, but a creature with an ability can use it as much as it likes.
So, Otyugh's Devour, an instant kill spell; Maxwells Paradox, another instant kill; and my favorite of all Fallen Druids's Improved Mutate, which gives you the ability to turn a random critter into a 1) better creature and 2) with an ability.
So, that's Gravity, Entropy and Life - for Devour, Paradox and Mutate respectively. But even then, there is a high chance that your Mutations are going to be one of the 9/12 elements you cannot use. So, a logically progression from a Trio deck (Gravity, Entropy, Life) would be to use Rainbow. Then, moving on from this, it is logical also, since Maxwell's, Otyugh and Druid don't cost very much, to expand your rainbow deck using other low cost cards... Quintessence, Plate Armour, Firestorm etc... Are all very handy cards to have in a deck.
Okay okay, so I've pretty much repeated what has been said before. But, admittedly I didn't read everything so don't shoot me!
What I'm getting at is, we need to either:
1) Vastly increase the cost of certain creatures/creature abilities
2) Give certain elements really high cost creatures with awesome element specific abilities.
Imagine if each and every Dragon had an ability which was just amazing? Typically a Dragon doesn't make its way into a rainbow deck - because they generally cost more than 10 quanta. How about if we say you can only use a Dragons ability if it is your mark.
This is pretty much what the Nymphs are. But, there is nothing really stopping a Rainbow player from successfully including them into his or her deck.