I think it is pretty obvious. even without allied support, that GW designed the new Genestealer Cult codex to have multiple synergies within the army, especially when you combine the various abilities of the HQ options, unit upgrades, and formation/detachment rules. Here are a few Genestealer Cult battle groups that stand out to me on initial analysis:
 Mechanized Assault Battle Group:
**8 Metamorphs with Metamorph Claws (one also has Cult Icon), Metamorph Leader with Metamorph Whip and Bonesword, attached Primus with Sword of the Voids Eye relic, all in Goliath Truck —
So, this unit is WS5 (Primus is WS6) as long as the Cult Icon bearer is still alive, with 32 x S6 AP- I4 Rending attacks, 5 x S4 AP3 I7 attacks, and 5 x S5 AP3 I4 attacks on the charge (and all the AP3 attacks have Instant Death on rolls of 6+ to Wound). On top of this, the unit gets re-rolls to Hit on the charge (from Hatred USR on the Primus) and can ride their Goliath truck 6″, disembark another 6″, and charge 2D6″ for an average threat charge range of 19″, making for a very strong chance of a Turn 2 charge. As you can see, this is some very potent close combat damage output that can threaten everything from hordes of light infantry to MEQs, TEQs, MCs, and even vehicles, with so many Rending attacks at WS5 (and with re-rolls from Hatred). The biggest threat to a unit like this is powerful Overwatch abilities, like Tau Supporting Fire, Skitarii Cognis Weapons, and Dark Angel Supreme Fire Discipline, since these Genestealer Cult units only have a 5+ armor save.
 Backline Firebase Battle Group:
**Take “The Doting Throng” formation with a Magus (PML(2)) and 6 squads of Neophytes (all with Lascannons), plus an allied Genestealer CAD with another Magus (PML(2)), an Aegis Defense Line (with Quad Gun), and 2 x 5 man squads of Acolytes (with 2 x Heavy Rock Saws each) in Goliath Trucks —
The way this works is both Magii roll hard on Telepathy for The Shrouding. All units are deployed behind the Aegis Line for a 4++ Cover save (which becomes a 2++ Cover save if you get The Shrouding off successfully). Based on the formation rules, all Neophytes within 12″ of a friendly Magus (even the CAD one) have Zealot, so they are Fearless. Between that and the strong cover save (without even having to go to ground), the Neophytes should be able to hold the line quite handily and keep shooting. In the event that they do suffer significant casualties, you can use Return to the Shadows to get them out of harms way and reappear via Cult Ambush later on (or somewhere else), where the threat is less dire. Additionally, the Goliath truck mounted Acolytes provide a “counter-charge” capability in case enemy assault units get too close and the two Magii can use their Telepathy abilities to further enhance the friendly Genestealer Cult units (or against the enemy, like using Psychic Shriek against enemy MCs). Finally, the Goliath Trucks provide some additional TLed Autocannon fire support and the Quad Gun, manned by a Magus, provides solid anti-air/anti-skimmer shooting as well. Taken all together, this battle group gives you a very solid, surprisingly durable “fire base” around which you can build the rest of your army (and you can use Cult Ambush/Return to the Shadows if you need to maneuver these forces to a different part of the board later in the game).
 Infiltrating “Shock Troop” Battle Group:
**Take the “First Curse” formation (Patriarch with PML(2) and 20 Purestrain Genestealers) as part of the Cult Insurrection detachment, plus an additional Patriarch (also PML(2)) from a friendly Genestealer Cult Allied Detachment —
Use Cult Ambush to set up the two Patriarchs (both of which roll hard on Biomancy, looking especially for Endurance) and the Purestrain Genestealers into terrain as close as possible to enemy forces as you can before the game starts. Between 5+ Cover, Stealth (from the Purestrain Genestealers), and Shrouded (from the First Curse units), the entire Battle Group should have a 2++ cover save turn 1. Add in Fearless, Unquestioning Loyalty (for automatic Look Out Sir on the Patriarchs), and either Endurance (on the whole unit) or Iron Arm (on one or both the Patriarchs), plus the 5++ Invul saves on the Purestrain Genestealers, and you should be able to shrug off A TON of enemy shooting on Turn 1. Turn 2, you can move forward and charge up to three separate units with your two Patriarchs and Purestrain Genestealers (with Fleet for better chances at making the charge successfully). This unit ought to make short work of almost any enemy unit they charge (even MCs/GMCs), due to their massed Rending attacks at very high WS and I. Once they are done, they can move onto other enemy units or, if necessary, go back into Ongoing Reserves (via Return to the Shadows) and reappear elsewhere on the board the following turn (this also gives them the chance to replenish any casualties they have suffered). Finally, you can make them even more effective if you can get a Primus within 12″ of the unit (he cannon join them like the Patriarchs can), since that confers Hatred to the whole unit on the first round of any combat. If you ever do manage to roll a “6” on the Cult Ambush table (either during deployment or later in the game), then you can have the entire unit arrive and charge on the same turn, making it even more effective (Note: if one of the Patriarchs is your WL and he manages to get the “Ambush Leader” trait, he and the whole unit can do this at will, without even having to roll a 6 on the chart).
 “Anvil” Close Combat Battle Group:
***CAD with Iconward (with Icon of the Cult Ascendant relic) and Primus (with Sword of the Void’s Eye relic) and unit of 8 Aberrants (all with Power Hammers) riding in a Fast Attack slot Goliath Truck —
The Iconward gives the unit +1 to its FNP (so Aberrants have FNP(4+)), plus his banner gives everyone +1 attack and Furious Charge… this means the Aberrants have 4 x S9 AP2 Unwieldy attacks on the charge. As for the Primus, he has 6 x S6 AP3 attacks (with re-rolls of 1s to Wound and ID on a 6+ to Wound) on the charge. Additionally, the entire unit has Hatred, making them much more accurate with their close combat attacks in the first round. The Goliath can reliably deliver this battle group into assault and the Primus’ Ld10, plus Stubborn from the Aberrants, can ensure they are safe from Ld shenanigans once they get there. Finally, the unit is Majority T4 (thanks to the Aberrants) and has 21 Wounds, which synergizes with the “Unquestioning Loyalty” rule for the ICs and the FNP(4+) on the Aberrants to make it surprisingly difficult to take down with anything that is not S8+ (i.e. its takes, on average, 16 x WS4 S4 Power Sword attacks to kill a single Aberrant). Taken all together, and you have a fast moving, hard-hitting, durable close combat group that can also be deployed via Cult Ambush, if you want to leave the Goliath at home.