Cancellation may not be essential, but I've lost way too many games due to the inability to cancel things. I'm not looking for anything nearly as simple and straightforward as Spirit's cancellation. I'm looking for something more akin to the new Lore event. It needs to be set up before you can cancel anything, and you're very limited in what you can cancel. Tactics and Leadership should have something similar. It doesn't need to be treachery cancellation. If it cancels something specific to enemies, that's great too. If it can only cancel something if you've destroyed an enemy earlier that round, then it'll only work in direct damage decks, and that'll be fine. Make it as situational as you want, but I think every sphere should have some kind of cancellation, especially when it comes to multiplayer, where the effects can potentially more dangerous and far more chances are given to the encounter deck to reveal its dark side. Heck just give tactics some shadow cancellation and skip the encounter cancellation.
Spirit excels at so many things. I just want each sphere to have a minor way to contribute more than numbers toward defeating the scariest parts of the encounter decks: card effects.