Hey there! I like how excited you are to work on your game! However, before you start back working on that multiplayer mode, I have a few simple suggestions to help make the game better in general and get people more excited about what you're doing with it.
First, I know it's easy to overlook this when you're testing your own game, but currently, there is no way to respawn after you've fallen through the world! You should think about whether you want the player to respawn automatically, or whether you want to have them press R to respawn.
Second, I think people would really like your game if you tweaked the graphics a little bit. Currently they look very basic, and I apologize if this sounds insulting, but they remind me of my days on scratch.mit.edu. If you can't download free assets made for game developers or remake them yourself, why not reach out to an artist on Newgrounds who might be able to help you?
And third, I would like you to take a bit of time to figure out what the objective of your game should be. I see in another review that the first level can't be completed, and I wonder if the problem is that you haven't come up with a thing the player must do to complete it. (If you have, I'm sorry for the assumption.) I know that as a developer, it's fun enough to just jump around and you easily think of it as a game in that aspect, but players, who don't have that love and connection to your game that you do, need to have something to make them want to interact with the world you've built. It doesn't even have to be going through multiple levels - you could instead have coins to collect that award the player points. This type of point-based format would translate very well to a multiplayer version!
I hope you found my advice useful and not insulting. Don't give up on this. It's got a lot of potential to become something really interesting. :)