Interesting idea! I like when a mission comes with specific list building requirements that encourage building your list in a fundamentally different manner from a normal game.
A point of feedback — while I don't think the Sudden Death rules are likely to become relevant in the first place due to the many ways the game offers for killing a hero, I think it's a bit suboptimal that they completely abolish the activation and action system. Changing/ignoring the core game rules on such a fundamental level might cause a variety of issues: it's not defined whether the heroes may use melee, ranged, or both kinds of weapons, they don't get a chance to use Caster or other special abilities, and possibly more. It might have been more elegant to just say something along the lines of, the heroes must spend their activations charging or shooting each other and can't move to be more than X" away from each other.