I'm working on making a converted C'tan-as-Omnissiah model. Whilst searching for cool pictures of other C'tan conversions, I discovered OddJob's AdMech. And by the Omnissiah they're awesome!
They're largely focussed on huge stompy robots & armoured suits, using Penitent Engine pieces heavily. Rules-wise they're (fascinatingly) Tyranids, with Techno-Magi as Zoantropes, servitors as Hormagaunts and big stompy walkers as monstrous creatures. Seriously cool, and a great colour scheme (pale slightly blue skin contrasting with the red cloth).
Beautiful & inspiring stuff!