So, your Rogue Trader and his ship are exploring the uknown expanse. Or are lost in the warp and come out in an unknown system. What mysterious and unusual planets might they discover from over 20,000 years of human exploration ?
A few that come to mind-
The Planet of the Navigators - home to Dark Age colony of 3 eyed Navis Nobiliate who'd fled here as the warp storms rose. As the only inhabitants, they have interbred for 20,000 years and now cover the planet. None have navigated the warp before, yet their ancient rituals and school learning still preserves the basics of Navigation.
The Planet of the Squats. Land Trains, trikes and Exo-Armour, a Squat homeworld of progressive Hereteks still survives and thrives beyond the Rim.
The Zoat Embassy- urbane, cultured, educated - and also reptilian, centaur-like Xenos advisors who are bootstrapping a mediavael human culture into an advanced space fairing race. Imagine Leonardo da Vinci as a mighty alien centaur with the voice of Frasier Crane. But have you heard that Zoats are the acceptable face of Tyranid assimilation?
Biomass Matrix 001. Its true that the Hive Mind is distributed amongst the Norn Queens and Synapse creatures of the Fleet- each comglomeration a contributing Node to the Hive Mind. But some Nodes are more equal than others, and Biomass Matrix 001 is the guiding force behind this fleet. You've just jumped into the same system as it and its many bioships. What do you do? Could you even negotiate or threaten the Hive Mind?