IANAL.
Gameplay ideas themselves cannot be copyrighted. If you need any evidence of this, just look at the wrath of clones of any specific popular game, such as Call of Duty or World of Warcraft, available on the market. You cannot use any of the original content, however, unless it's independent- for example, just because CoD uses an AK-47 doesn't stop you from using it, but because WoW has a race called the Night Elves pretty much does preclude you from doing something identical.
The general rule is that if the gameplay happens to be similar to an existing game at the end, then that's one thing, but if it's a straight rip of another game from the start, then you're not going to win many fans and may end up in court.