I don't think that would be necessary. I can only speak for myself, but personally, I have no desire whatsoever to see Bumper, Sissy or Gill next door get any more screen time, let alone any fleshed out appearances. I don't give Johnny Test a lot of compliments, but I actually find it to be somewhat admirable how the series has managed to stay afloat for 4 years without feeling the need to rely on gimmicks such as bringing in a bunch of new characters. Test has basically been the same show, with the same emphasis on the same characters since it's beginning. Same deal with Ed, Edd 'n' Eddy, which went throughout it's entire series run without ever introducing a single guest character, save for the finale, in which Eddy's big brother appeared on screen for the 1st (and only) time.