One of the nicest elements of the Puzzle Quest formula is mana denial. The tutorial even references it.
But in this game, the best move is the one with the most cascades, with mana color being merely a tie breaker. Limiting your opponents’ opportunities for cascades is rewarding and challenging, but it’s not nearly as central to gameplay as mana denial in other PQ games (and derivatives).
I’d suggest making mana denial more important by requiring a particular color to charge your cards. Dual color cards would come at a premium. As with other PQ titles, when there are no advantageous matches for your hand, you can focus on either clearing advantageous matches for your opponent or take the risk of making cascades to introduce new gems to the board from the top.
Color-changing supports and abilities would be more important for increasing the availability of your colors, rather than simply creating a mana-gain lottery each turn. Elvish visionary would become playable.
You’d want to make the card colors of your hand visible in the collapsed view. I’m not sure, but I’d consider doing this for the opponent’s hand as well (maybe only for partially charged cards?).
You could also allow off-color cards in a deck. Gaining one or two less mana on a match is a big deal with this change, keeping planeswalker decks diverse, with more potential for mix-and-match strategies. Although you might want to set the baseline to +0 on-color, -1 off-color, -2 enemy color.
While you’re at it, you might consider allowing full 40 card deck construction, providing 4x of each base card when you unlock a planeswalker. While it’s true that you reduce the ability to build a deck around a newly acquired rare, with the off-color change, you get a lot more interesting options for incremental enhancements when you open a pack.
The flood of Nissas in the leaderboard creates something of a forced metagame. While it would complicate the tutorial implementation, I’d suggest allowing players to pick their preferred planeswalker at the start, and then increase the cost of buying planeswalkers, commensurate with the increased value of their library of base cards with the off-color change.