What type of system do you feel comfortable with?
Do you want simple? Crunchy? Classes? Levels? Point buy?
I could always recommend Basic Role-playing System from Chaoisum. All races types are made the same. They also have Magic World, which is their generic fantasy rules. Considering Call of Cthulhu, Stormbringer, Runequest, etc all use the same rules, you could allow the players to think they are playing a modern set game, and then BOOM! Another world, different humanoids, etc.