Now that I've had a few days away from it... I think a big improvement to the game design might be made if I changed it so when you put your opponent into check, they then get a "free turn" to stop the check (under normal check rules). This would avoid the king-queen chasing and endless ties I keep having in my own tests, or the queen mopping up your pieces while you move your king as you would lose otherwise. Essentially making checks resolve like normal turn-based chess for a round. If a check-feature was implemented in this game without that change, the game would be much worse, as you'd have to block your king, move your king, or attempt to take the threatening piece, while the other piece safely killed everything else on your field.
It would be a fair more work of course, but I think that change could really improve the game, actually...