I think a Custom Mode and a multiplayer mode would be great. In custom mode, you could alter the health (how much health you both start with and when no-defibrillators activates, if at all) and items (if you get items or not, what items you can get, and how much you can pull out per load) as well as more to customize. In multiplayer mode, you would fight against another player and play 3 rounds, the best of 3 wins. Those are some of my ideas for potential new gamemodes.
Very fun game with a lot of replayability at such a cheap price.