How do I create a spiritual game successor?

An anonymous reader writes: So I have recently been on a legacy video game binge, reliving the nostalgic days when I realized that one of my favorite old games can be vastly improved with a few tweaks. This game is pretty much made for a controller, so I would love to get it done on Xbox Live, but doing it on the PC is just as viable. Unfortunately, I am pretty positive the game is not in the public domain yet. Based on previous stories covered here, some companies are all for community made successors while others choose to give them the crushing blow from the start. My question is, how far is too far when one is trying to make a spiritual successor? FYI, I do not intend to copy any materials, but it would be lovely if I could incorporate some game design ideas (very general level design, movement, and just one or two game features).