The concept I've been working up is an online platformer that would allow up to 3 or 4 players to explore the game cooperatively.   

The main gameplay would center around the Metroid-vania exploration formula, but with the added twist of being able to play with a couple of buddies (Though the game could be completed as single-player as well). This would allow players to split up item collection and team up on difficult bosses.

Example: There is a roadblock impeding the players progress until the hammer is found in the deep recesses of of the map. Rather than having to stick together, the players could go off exploring for the hammer by themselves, then team back up to finish off a higher leveled boss when the item is found.

I've been working in an HTML5 engine called Melon.js (http://www.melonjs.org) and coding the server side in a technology called Socket.io. The game is written in web code, so it should be compatible as a Facebook game, Ouya, iPhone app, Android app and standard web browser game as long as we have a compatible wrapper for each platform. There is also no need to download any desktop software when running the game, as all the code and art assets are stored online.

So... Now that I have the basic online play proof-of-concept I am starting to turn my attention to gameplay, art assets and sprite-sheet animations. The map builder that would need to be used is http://www.mapeditor.org/ , where there would be some overlap in terms of tilesize, dimensions, etc. You would not need to know any code, but would need to have a basic understanding of what names and properties I am using for player / enemy / collectable objects within the maps.

As far as spritesheet animation, I am not expecting anything crazy intricate.. I think a retro NES look would cut down on development time and still have a nostalgic appealing look. Terraria, Simon's Quest and Mega Man are games that come to mind for what I had in my head. There is a wide variety of spritesheet animation software out there and I would be happy to do research into what would fit the project.

As far as I'm concerned, you can have as much artistic freedom as you'd like. My only sacred cow is that it follow the Super Metroid / Symphony of the Night gameplay formula w/ online play. I would also consider art vs. programming to be a 50 / 50 split stake in ownership. I have not fully thought through monetization methods and I am open to ideas.

Sorry to flood you with information. I wanted to try to give you a fairly comprehensive overview of what I have in mind for the project. If you would like to see a basic demonstration of the online capability (there are still a few bugs) let me know and we can do a chat.

Thanks for your time!