You could always be fancy, and use the MS Scripting parser thing (and let people write in VBScript or JavaScript). That way all you have to do is provide a bunch of objects for use by the script. Whatever's used should be something designed for this, and not something like IRC syntax (which has its uses but not as a scripting language).

I'd suggest at a minumum Game, Player and Event classes. You could then add subclasses like Baddie, Boss, Weapon, Shield, Food and so on.

Game would be the grand-daddy of them all, and do things like manage game state and provide a method for accessing and adding Players and Events. Player would contain access to the state for a specific player, like health, ammo, gems, coins, etc. Event would contain the event params and location. Child classes of Event could have more specific details, like the current health and location of a baddie.