Part of the problem is the rules as written only allow Astra Militarum forces to be taken as Brood Brothers with a Genestealer Cult list, and you're not allowed any named characters. This is compounded by the fact that for ninth edition, Games Workshop seems to be actively trying to move away from the "soup" meta, also known as multiple detachments, which was common during eighth edition. In White Dwarf 459 from December 2020, there was this question:
"I've been wondering for a while if the Genestealer Cults could take over an Imperial Knight, or even make their own. They they can, why don't they?"
Games Workshop responded with an answer indicating that the GSC couldn't use the interface and/or an infected Noble would be rejected by the Knight, but the overarching actual answer was clear: "No, we're not going to open up the full 'Imperium' key word to the Genestealer Cult." Which as a Genestealer Cult player was a somewhat unsatisfying answer.
So what if we want to throw out the official Games Workshop stance and make a fully realized Imperial Cult army? What would be stopping us? Well, honestly nothing as long as one is willing to make the conversions and follow the rules as written. If you wanted Genestealer Cult Knights, you could technically do it as a multi-detachment force. The catch is now you're building an army with the code word Imperium, not Genestealer Cult.
|Looks like a Vindicare Assassin to me!|
The upside of this, is now anything with the Imperium keyword is fair game for conversion and insertion into new force. That includes Knights, Assassins, Space Marines, Sisters of Battle, you name it. Over 300 units are available to the enterprising list builder and converter. The downside is that many of the Genestealer Cult units themselves will have no home in the new list as they will have incompatible keywords, but many cherished units could easily be ported over. For example, a sniper Sanctus could easily serve as a Vindicare Assassin or a Magus could easily serve as a Primaris Psyker.
Ultimately it really depends on how far down the rabbit hole you want to go, how many conversions you'd like to make, and how many incompatible units you want to generate that can't be played with the core Genestealer Cult force. That being said, I'd love to see someone do a Cult Space Marine chapter, but I can just hear the screams of Heresy now!