Excellent game! I wish there were similar games out there, I love the simplicity yet the challenge that comes with it. Just by adding a background to the menu and textured buttons and it could easily make its way into one of the best simple indie game I've ever played.
Adding multiplayer could be an actual challenge, the ping difference would be much more noticeable when you only have a fraction of a second to counter-attack a block or dodging, but it would definitely make it easier for friends to play together, even if not competitively.
I can't wait to see what you add on future updates!