Now that Traitor Legions is officially out (and is an outstanding release overall, I’d say, breathing whole new life into both the tactical abilities and rules-reflecting-the-fluff aspects of the 9 original Traitor Legions), I think there is a pretty obvious shortfall that needs to be addressed whenever the “base” CSM book finally is updated: the obsolescence of Cult Troops.
Given that even basic CSM now get buffs to bring them into almost complete parity with Cult Troops (i.e. Khorne CSM now get Fearless, Furious Charge, and Adamantium Will for “free,” meaning that they have everything Bezerkers have except WS5/access to Chainaxe upgrades and cost 4 points less per model; or, Nurgle CSM also get Fearless, FNP, Relentless, and -1I, just like Plague Marines, but cost 8 points less per model, with the only compensation for Plague Marines being their Blight Grenades, Plague Knifes, and access to 2 Special Weapons when at minimum squad size; etc., etc.), I think GW really ought to provide some improvements to the 4 different iconic Cult Troops to make them worth taking, and not just providing some special formations that give them buffs like they have already.
Based on that, here is my idea for how to improve Cult Troops in a reasonable manner (while keeping their current points costs the same) so that they are not auto-take, but also are still a competitive option as compared to basic CSM with their respective Marks:
 Khorne Bezerkers — Give them back their old 2A base (so, that would be 3A base on the Bezerker Champion) and make Chainaxes a S: +1 AP4 Rending weapon.
*Rationale — Between WS5 and 5A on the charge, I think this would justify the 4 points/model higher cost of Bezerkers over basic Khorne CSM. Additionally, if they paid the extra 3 points/model for the Chainaxe upgrade, not only would they gain +1S and AP4 (so S6 on the charge), but the addition of Rending really increases their versatility and overall close combat hitting power, making them a more expensive, but more capable, close combat option to Khorne CSM (which is what they should be, as an elite unit).
 Plague Marines — Increase their FNP(5+) to FNP(4+) and add Poison (4+) Bolter ammo to their Wargear.
Rationale — At 8 points/model more expensive than Nurgle CSM, Plague Marines need a bit to justify taking them over their less “elite” counterparts. Based on this, I think they definitely need more than Blight Grenades and Plague Knives to set them apart from basic Mark of Nurgle Marines. Increasing their FNP one “step” I think would give enough of a boost in survivability to make them a strong option for taking/holding Objectives and giving their Bolters/BPs Poisoned (4+) ammo makes them kind of like a CSM version of Sternguard. Overall, with Defensive Grenades, Poisoned (4+) for both close combat and shooting, and enhanced FNP rules, I think Plague Marines would be worth considering over “normal” Nurgle CSM.
 Rubric Marines — Improve Inferno Bolts to (S:4 AP3 Armourbane, Rending), improve Soulreaper Cannon to (S:5 AP3 Armourbane, Rending), and improve both Warpflamer/Heavy Warpflamer to Torrent (6″) weapons (this would obviously impact Scarab Occult Terminators as well).
*Rationale — Like Plague Marines, Rubric Marines are MUCH more expensive (8 points/model more expensive) than their Tzeentch Marine counterparts, but also are a lot better than them out the gate (i.e. 4++ instead of 6++ save, AP3 Bolter ammo, access to Psychic Powers via their “sergeant,” etc.). What Rubric Marines really need, I think, is better shooting versatility, instead of being limited to just AP3 Bolters. Adding Armourbane to both the Soulreaper Cannon and their Inferno Bolts makes them MUCH more versatile against light-medium vehicles and adding Rending to the Inferno Bolts gives them a chance at dealing with Terminators, Heavy Vehicles, MCs, etc. Finally, making the Warpflame weapons into Torrent (6″) gives much more reach and versatility for these anti-horde options, especially since Rubric Marines are so slow outside of Rhino/Land Raider transports.
 Noise Marines — Give them BS5 and allow them to take one Blastmaster (30 points) or Doom Siren (20 points) per 5 models in the squad.
*Rationale — Noise Marines are currently only 2 points/model more expensive than their Slaanesh Marine counterparts, but they also have nothing to show for it, except access to Sonic Weaponry (which they have to pay even more points for, and Slaanesh CSM have access to normal Special/Heavy Weapons that Noise Marines cannot take). Giving Noise Marines BS5 would justify taking them instead of normal Slaanesh Marines and would make purchasing the Sonic Blaster upgrades even more worthwhile. Also, giving them options for up to 4 Blastmasters/Doom Sirens in a max squad of 20 (or 1 in a squad of 5, 2 in a squad of 10, etc. etc.) would help “customize” them as a solid option in competitive play.