The Viper Legion are a loyalist chapter of Space Marines. Only three things are known about them aside from their name:
1) They are based in the Aurelius System.
2) They're part of the Astartes Praeses.
3) One of their sergeants got killed by Hecaton Aiakos in a friendly fire incident once.
Considering they are called the Viper legion, snakes probably have some role in the icons/colours of the chapter. If that is the case, perhaps they will end up looking like space versions of the snakemen from the Conan the Barbarian cartoon. Or a sci-fi version of Warcry's Splintered Fang warband.