There are many threads about the unfair strength of rainbow decks. The main point that seems to be raised is that the game should be as interesting as possible. Many people want to see the game go in a direction where single and two colour decks are not marginalized by rainbow decks, which is the trend. The problem with trying to rebalance things, is that it's uncertain whether the game's interest will be improved for most players-- maybe a simple one colour deck will dominate and a new balancing will have to be done.
My solution complicates PVP a little bit, but it would make the game immune to unbalanced decks. It would increase match time though. The idea is to play matches in groups of 3 with each match being worth twice as much as the last. Between matches, you can add/remove/change up to 15% of your deck. Now, when you see someone is playing a rainbow deck, you can add some quicksands or whatever to counter it... and so on.
It would make the game a little bit like rock-paper-scissors though.
