Defining and testing custom made DFT functionals?
Depending on what you're after, you might want to look for programs using libxc, so you can completely define your own functionals.

From the libxc section of the Octopus wiki, these include:


  
  Abinit – plane-wave code
  APE – an atomic code
  Atomistix ToolKit – numerical orbitals code
  AtomPAW – projector augmented wave functions generator
  BigDFT – wavelet code
  CP2K -  A program to perform atomistic and molecular simulations of solid state, liquid, molecular, and biological systems.
  DP – Dielectric Properties, a linear response TDDFT code
  Elk – FP-LAPW code
  ERKALE – a DFT/HF molecular electronic structure code based on Gaussian orbitals
  exciting – FP-LAPW code
  GPAW – grid-based projector-augmented wave method
  JDFTx – plane-wave code designed for Joint Density Functional Theory
  MOLGW - a small, but accurate MBPT code for molecules
  octopus – real-space (TD)DFT code
  Yambo – solid state and molecular physics many-body calculations code
  


On a more personal note, and this is just my opinion, if you're just arbitrarily mixing functionals, you should really make sure you have a scientific justification for doing this, otherwise you're just generating numbers without basis.