Instead of gaining mana from matching gems on the board, maybe some troops could only gain mana from 4- and 5-gem matches (i.e. on extra turns). If these troops could also charge during the opponent’s turn, then it would offer the AI a bit of a counter to control teams.
Edit: dang. Didn’t think about how this would work with traitstones. This is left as an exercise to the reader.
Some time ago I was thought about “spirit” troops (whole kingdom) which troops could be 0 attack and collect mana from skulls, like new mana type.
The other idea could be to make them traits. Make a standard trait that gives 1 mana per turn. Then there could be a legendary mana generator that is an Adana construct with a legendary trait of +3 or +5 mana per turn whose ability would give other troops mana in some way. Similarly, there could be a troop with a legendary trait that gives 2 mana to all allies per turn.
You’re right that traits would fit better into the current system than my colorless idea. But a mana gain per turn doesn’t address the issue that control decks punch way above their weight because the AI never gets a turn.
The next kingdom is likely to have a lot of freeze. I wouldn’t be surprised if the legend there freezes on 4x and 5x gem matches. The kingdom may even get a freeze all or freeze 2 ability.
Woof. The thought of a multi-freeze spell gives me chills. And not the fun kind. Would much rather have a troop that powers up during my extended turns. Such a troop would still never get to fire off its spell if everything goes as planned for my team. 
With the whole time thing as a mana type why not let it gather using skulls on matches. It would then fire off a one time spell that is really powerful. Would be interesting to run a team based off of skull mana gaining. The troop can only gain mana from skulls if it does damage so you can not gain outside of matching skulls with say a skull generator. Also using match 4 or more would mean that most matches would be drawn out to get the spell and there is the common complaint of matches taking too long. This way the matches can be quick but also no new gems need be introduced and the mana gain can not be abused, nor the spell in question.
How about a legendary trait that with every +4 or +5 match the opponent makes, it gains 1 mana.
Maybe it could be a legendary monk of sorts, and the Trait would be called Meditation.
Would be the first Legendary Human troop too. I’d like to see that.
That sounds amazing. You can call his spell “enlightenment”
After more thought, having it be a trait makes much more sense than trying to finagle a new color mechanic into the game. Not as much of a fan of it being legendary-only. I would like to see several troops, maybe even a kingdom, built around the idea of using opponents’ 4- and 5-gem matches against them, and they can’t all be legendary in that case.