Zoats! Psychic ambassadors/spies of the early Hive Fleets, either engineered or enslaved (or enslaved and retro-engineered) to undermine and assess the galaxy ahead of the Tyranid arrival. 

Functionally independent of the Hive Mind, we can assume they acted as a 'symbiotic' species for the Tyranids, like pilot fish for sharks (or the Silver Surfer for Galactus), up until the Hive Fleets determined the relationship had outlasted its benefits, and began reacting to absorb their biomass... a fate which at least some Zoats escaped, such as the Forerunner and Archivist, who both made their way to the Blackstone Fortress.

A symbiotic, semi-independent species that serves the Tyranids until being consumed... sounds familiar? 

I think Zoats would fit perfectly into the Genestealer Cults, having learned to 'hack' each Broodmind's psychic IFF to recognise them as friendly heralds of the Star Children. Some of them might act as wandering 'mercenaries', passing through and using the Cult as a safe haven before moving on once their goals are achieved. Others might try to outright usurp effective control of a Cult, positioning themselves as  long-term 'advisors' to the parasitised Patriarch.

I wouldn't want them to be a major chunk of the faction, but Zoats could add a new visual angle to the GSC roster, expanding them beyond "space miners", "xenomorph mutants", and "Illuminati cultists". They'd also potentially open up more outlandish alien technology, contributed or adapted from Imperial tech by Zoat technologists.

Add Zoats to the Genestealer Cults