You have my vote on that.
Starting it as a hack gives strict guidlines that people can follow, which will drasticly cut back on all the bullshit of "Wouldn't it be amazing if we could do this?" or "I have an idea, -insert insane nonsence-".
Also with the level layouts handled like they would be in a hack, level designs can be more focused, put together properly, with designs for actual tiles to scale. Rather than doodles on a piece of paper.
Once thats done then maybe you can think about expanding upon the crazy ideas. And if it gets that far, then and only then think about moving beyond the limitations of the Genesis.
Pace yourselves, don't get carried away too soon.