Now I like the Idea of Genestealer Cults, but I saw the reason why they didn't include them back, or more specifically why Guard cannot ally with 'Nids... because it be power-gaming to the max, it would literally say hi to the Meta and then proceed to tear it to bits, it would be at the top of the Meta and everybody would get them just to win tournaments.
Now while I can accept the Idea of Genestaelers with Guard, if 'Nids were ally's with Guard (and/or the other way around) the reality would be that Most people would have Tervigons everywhere while having a squadron of Basilisks or a Manticore at the back, or some use of Barrage while having a platoon squad with Orders given to it while having Tervigons giving berth to Gaunts 24/7...
so you now have
AV to those
MC... and Barrage... and Vendettas...
While Genestaelers Cults are a great Idea (I was not with this time as well I started in the end days of 4th) it would lead abuse like none other if they were ally's with Guard, and players (especially on the Internet) just love to exploit rules and abuse the system (not saying anything bad about it, just it can get a bit tedious seeing the same thing rule with a Iron fist over and over again).
The only way to have Cults back is to bring them in the Next 'Nid codex so it can tell them what they can units they can bring and re-introduce them as new units for the codex giving you another new option of list making (by that I mean a new list such as 'Nidzilla, swarms, and then if it were too happen, Cult lists, also the Idea to bring back Limos for the Cult leaders as a Dedicated transport would be awesome

).