I believe you can also get control of a Purestrain Genestealer for the first D3 rounds of a game (then it stalks back in the shadows), from the GS Favour table.Purstrain genestealers are back as a scenario tool called horrors of the underhive. Basically adding an arbitrator controlled of player roll off for control NPC. Huber You can also have a possessed hiver or a zenos abomination which a variety of stay lines.
It's a very nice toolkit for generating undeehive gribblies.
Lots of interesting scenario particularly the multiplayer scenarios. I particularly like the idea of the "Hunger games scenario" where you three lowest cost fights take part but they are unarmed and must scavenge weapons from crates.