My Thunder Warriors have to have a big impressive centrepiece, someone to lead them who can count as Logan Grimnar or Abaddon or a Demon Prince. So armed with a power fist, a power sword, and a big bolter... The person who led the Thunder Warriors, obviously, was the Emperor. GW hasn't made a model for the Big Man, and none of the 3rd party models can be used at GW events, so I really wanted a suitable GW model to convert. For a while I flirted with using one of the Primarchs, then I remembered Inquisitor, the GW 54mm game. Captain Artemis was a possibility, he's a good Space Marine to base things off, but ultimately I ended up falling in love with the mixture of elaborate detail and brutal power that Witchhunter Tyrus has. This is the Emperor a few centuries before the classic GW paintings, the Emperor when he was unifying Terra not leading the Great Crusade or facing down Horus.
Some more musings on the model here