Yeah I'll have to admit... I liked mitosis as a theme more than momentum so I kind of ignored the main theme. I tried to make it work with the idea that you need to set up good plays early on in the battle or you'll be punished for your past mistakes, but that only makes sense abstractly. I think momentum can be interpreted as "once you make a move, you need to commit to it, because there's no going back."
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Maybe something where you can only place cards on the back/leftmost column and they advance forward toward the enemy every turn? I'm sure that would take a lot of rebalancing and it's a bit late for the decision, but that's where my mind first went for how to more literally display momentum/inertia in this context.