
My starting point for this list was "Space Marines turned up to eleven". The first draft was probably a bit over-the-top but thanks to feedback on the Forge Of War ITEN forums it's now a bit more balanced, but the Custodians would still give the Astartes a good arse-kicking mano-a-mano.
Check out the Adeptus Custodes Retinue List (docx format).