Well, if you're building a codex, my suggestion would be 3 tiers of "troop". Rabble, Soldiers, and Elite.
Uncertain worth is random for the sake of random. A rule to differentiate them from being Guardsmen. A needless rule, in my opinion. There's no way that renegades would be nearly as disciplined as Scions, much less more so. Given that Scions have mediocre Ld in the grand scheme of things, I can't see why cultists would ever be more resolved. And how does one keep track of that through the game, do you roll before every Ld test?
Rabble would represent mutants, disgruntled workers, cultists, avid hunters, etc. 4 point Conscripts, with the option to exchange a Lasgun for either a Shotgun or LP+CCW. Or Autogun... whatever.
Maybe a 5 pt upgrade to take either Heavy Stubber or Flamer. A little pricier than 'Scripts, a little cheaper than CSM Cultists, but also a little more options. Chaos marks could be an option, but I'd avoid Icons. Too "elite". To offset the access to marks, you might want to give them improvised armour, instead. I see precious little difference between a 5+ and 6+ save, but it would be something. Squads of 10 to 30. Make sure there are "cheap" IC's that can join them to raise Ld. Priest / Commissar idea.
Soldiers would be traitor Guard / PDF. Same weapon options as Infantry Squads, except that you remove access to "energy" weapons other than Lasguns. No Plasma, Melta, Lascannons. *IF* it would be fluffy for PDF to have Autoguns instead of Lasguns, then you could see them as more or less modern infantry, using propellant based solid ammo weapons. I feel that at the "grunt" level, the expertise to maintain such personal fire arms probably wouldn't exist, or would be redirected towards maintaining vehicular weapons. They're more important than the grunts anyhow.
Option to exchange long arm for LP/AP + CCW. Access to Marks / Icons would give them an "alternate" advantage to Guardsmen, since they can't take energy weapons. They're more or less equal to Guardsmen, with equipment restrictions but access to different abilities. Perhaps a little better than Guardsmen, but without Orders. Orders can't be part of the Renegade list. They're free-thinking men, not auto-responsive machines like Guardsmen.
Elites would be based off of Veterans / Scions. These are the special forces guys, the high end mercs, the jaded ex-Militarum guys with a grudge to grind. Make them a 75 point base unit, with access to a choice [one] of Deep Strike, Scout, and Infiltrate. Access to the Veteran options, but a rule that no IC's can join them, and they can never benefit from Ld effects from other units. For example, they couldn't benefit from a Lord Commie's AoD, or a Warlord trait giving Fearless to all units within 12", that sort of thing. Call them "Hard Cases" or something like that. I wouldn't let them take Carapace upgrade, but the Camo option and maybe the Demolitions option would be reasonable. Access to Marks and Icons for sure.
"Super Mutants" could be something along the lines of Ogryns / Spawn. They'd be a fun elite choice.
Something along the lines of Psyker Battle Squads would be a good idea for a sorcerous Kabal. Or just unchecked psykers. They could be cheaper / more powerful, but a Perils of the Warp effects them all, since they're untrained / unprotected.
I like the idea of a lighter transport for them. Either a repurposed "truck" or the kind of military "cloth back" trucks that you see in ye-olde wartyme movies. That could be "open-topped" with maybe a Heavy Stubber for armament. Not fast, but cheap as chips. 10 man capacity prevents an IC from starting embarked with a squad. AV 10, open-topped, 3 HP, transport. Cheaper than an Ork Truck [25 pts?]. 3 HP on the basis that most of the vehicle is unnecessary to it's continued function. So long as the engine and drive train were armoured, the rest of the truck could be blown to pieces and stay functional. Who needs a roof anyway? Keep access to the Chimera, but not Taurox. That feels like special forces gear, most suitable to the Tempestus.
Keep Leman Russ options. Keep the Bassie, and Hydra. I'm not sure how common Wyverns are, but the mention of extremely sophisticated targeting systems would probably be hard for rebels to maintain. I'd want to remove them. Also remove Manticores and Death... uh... the giant Nuclear missile platform. I never use the thing, I can't remember what it's called.
No air support. The whole idea of keeping the Navy / Ground forces separate is to prevent wide-scale rebellion. Unless they want to cobble together their own Dakkajet type creation. Something less armoured, meant as a dog fighter. Lots of S6 shots would feel fluffy, while not threatening Imperial Flyers overly much. Not Imperial sanctioned flyers, anyhow.
Sentinels feel like a good fit. They're practically renegades in the fluff anyhow.
Access to "Chaosy" stuff would help to balance any restrictions on typical Imperial equipment. I've only really considered Traitors and Rebels here, without much thought towards Daemonic influence. Possessed units, or Daemonhosts would be suitable as well, as might access to a generic summonable daemon unit, and possibly a larger, non-greater Daemon. These guys are still small fish compared to CSM, after all, so probably couldn't handle the summoning of a proper greater daemon.