Here's the problem.
1. Fallen Gods are about the only reasonable source of income in the game.
2. All the advantages of Fallen gods lead themselves as rush decks - more cards + more quantum = faster.
3. Rainbow decks are the only decks that can stop the rush.
4. But rainbow decks have become so popular that it is stifling all competition, hence the need to nerf them.
I propose a solution to your dilemma.
1. In a game where you have 12 different elements, or mana types, How do rainbow decks play every type of mana every game? Answer: quantum pillars.
2. It comes a situation where dual or tricolor decks are actually inferior to rainbow decks, as quantum pillars can be used as lands as well as any mono pillar.
3. You suggest nerfing the quantum pillars. This does not go to the root of the problem.
4. I suggest rather than actually "nerfing" the mana generation, you should actually change the type.
5. Easy way: change the Quantum pillar from a pillar type to a regular card, limited to 6 per deck.
6. Really, the only "land" types where you can have unlimited number in your deck, should be the ones that produce a single color.
But, this will bring in a lot of cries. If you want to maintain the current level of farm:
Change the fallen god mechanic. Vary it up. Instead of having each fallen god draw extra card and get extra quantum, mix it up a bit.
some ideas:
1. heal itself to full every 10 turns
2. start with 3 random creatures
3. start with 5 quanta of each type
4. all permanents owned is indestructible
etc etc.
Also, you want more variety in decks, right? It just means you want more possible combinations people can play with.
there's two ways.
1. Make more cards. Simple as that. The more card types there are, the more decks can be made.
2. Make more ways for cards in different elements to be used with each other. Right now, other than rainbow decks, it's very difficult to maintain any more than two quantum types because there are no multicolor, less "random" pillars.
how to fix it?
1. Add "dual pillar cards," cards that are limited to 6 per deck, and can add two quantum of any combination of three elements.
example:
"Land name"
Add either Air and Water, Air and Gravity, or Water and Gravity.
You see where I'm getting at? it will make making tri or quad color decks much easier. It will not heavily change the game to favor multicolor decks, as you can only have up to 6 per deck.
Thanks for your time.