Chose a non-standard color scheme. I've managed to pick up a few Stormcasts without realizing it (between Warhammer Quest & the AoS start box), and thought it would be fun to try a custom Stormhost/color scheme. I also have a Knight Questor painted in the same scheme (so far).
The Stormhost is tentatively "The Order of Sigmar's Temple". They are tasked with recovering Sigmar's holy sites across the Mortal Realms, guarding those holy sites, and protecting pilgrims that travel between those holy sites. They are made up of those heroes who fell defending holy sites throughout the realms during the Age of Chaos.