Originally Posted by jotd
I think nowadays the future of game expansion lies in game remakes. I mean not game resourced to be ported to other plaforms (GBA unofficial Another World, Car Vup versions, Reminiscence aka Flashback remake from original data files), but games rewritten not using the original code at all, and with the help of a powerful editor to first recreate the original levels, and then to create new ones.
Game maker would be a good tool for remakes. It comes with a room editor that allows you to easily place tiles and objects. At the moment it can only make .exe but there is a mac beta so that will come soon.