Of course, you can't simply break one of the classes by creating a mini-nuke launcher for them -- there's a number of restrictions to what's appropriate for the game. Check out Valve's handy "Tips and Tricks" page to find out how to ensure your homebrewed creations are TF2-acceptable. For instance, a new fedora for the Sniper? Absolutely. A gun shaped like genitalia for the Heavy? Absolutely not.