I'm actually currently working on a multiplayer 2d side scrolling game right now as a personal project, so I'm very in tune with doing 2d stuff right now!
Although I'm coding my personal project in pure Java (which I could do yours as well if you wanted actually and it could still be browser based), I can easily make it in Unity as well.
Sometimes people are judgmental with Unity and say it can't make good games, which is why I generally steer away from it, but making it in Unity is not a problem, just letting you know I can do it in whatever! Giving you options :).
Either way, I have absolutely NO PROJECTS right now that I'm working on, so I could get this done VERY quickly. I'd bust my butt to ensure it's done quickly, and efficiently. I could even screen share with you while I code it if you care to see :).
I'd love to take this project, and make it a quality piece of work!