Eggbert, that's a neat idea! Though I will caution, engines like these need to be easy to use and extensible in very user-friendly ways. I myself may have programmed in JS for years, but I learnt it wrong, all wrong.
I should also say that, uniquely, we are at a position in which you don't need to use the default Map Engine anymore. If you can code the game outside of it, then the better it may turn out to be (and the more 'portable' it becomes).
And what do you mean you learned it wrong?
The reason I'm using the map engine is because it's part of the reason it would make things easy to code in Sphere. It has tiles, sprites, event handling, and other goodies that would be difficult to set up in other things. Why reinvent the wheel when you've already got a fairly good one?
Though one issue I can imagine might come up: Arcade beat 'em up levels can be fairly large, even before transitioning to new ones (like downtown to Discount Graveyard). Is there an easy way to have the engine load a new map without any pauses, jumps or anythin like that, keeping the players and enemies in the same locations?