Could you make it so that the longer someone plays, the harder it is to stay alive? Armour provides progressively less protection, you require more food, there are more monsters, monsters are more powerful, lightning more frequent etc...
It was something I had discussed with some friends who were testing it, but the combination of "would require more work" (although not necessarily too much-- there are various mods based around "survival" concepts that I could theoretically use) plus "gets away from the basic concept of out-of-the-box Minecraft" left me unmotivated to continue.
