I think it would be interesting to experiment with variations on the number of cards drawn each turn to see how it affects gameplay. This could be made possible with a new field in the custom scenario editor that takes an integer between 1 and N, for some large N. (I suspect that 1-card would lead to a more positioning-focused long-term game, while 3-card draw could be great for 5-minute blitz matches.)
Along the same lines, I'd like to try varying the number of cards you keep at the end of the round from zero to 5. Lower would make armor less useful of course.
