My 2 cents:
I with a few others spent a fair bit of time working on Horrors/Tzeentch for both Stunty Leeg and Secret League for FUMBBL. The concepts proved very difficult.
Mutations
Firstly for theme of course, you're right that the niche for this roster should be Mutations. However their thunder is mooted due to the fact that all chaos teams get mutations as regular access since LRB 5 onward. I'd like to see that reversed, however that may be a step backwards due to double access favouring coaches that can put in the game count......something the BBRC wished to counter and a problem with LRB 4 and previous.
So with that in mind....is there a place for a Tzeentch roster? I hope so.
If the mutation access does stay open as regular acces's on all chaos rosters (something I'm not really a fan of) the answer could be to either add some extra 'Tzeentch' mutations....bring back RSC, Spikes or how about the mutation table as a double access?
viewtopic.php?f=16&t=42084 I use this table as a double access for teams with a high rate of mutation.....it's a lot of fun.
Daemons
One thing we requested for Stunty Leeg that never happened was a new special rule for Daemons.........something the Khorne roster badly needed IMO. With a new neg trait, you can make Daemons with better stats. What I wanted was to bring back instability. This would work quite like blood lust, however you cannot bite anybody, but a failed roll doesn't cause a turnover.
Horrors
Horrors are a real toothache. So much so even GW/Warhammer scrapped the pink/blue horror concept (which is a bit of a shame). We spent weeks trying to think up a workable concept for Pink/Blue horrors for FUMBBL Secret League. One of the main hurdles there though is client limitations, which is something Table Top doesn't have. It's so much easier to come up with some cool concepts for the Table, that the computer cannot handle.
My suggestion would be that you cannot buy Blue Horrors. Instead when a Pink Horror is killed you get 2 Blue Horrors. Now the problem with this idea is A: how to make this beneficial and B: how to make sure it's not broken.
So a blue horror has to be a player you want to get for your roster, but again isn't broken. Either way it's going to favour the concept that the BBRC wanted to move away from (that being the more games you play, the more beneficial to your team).
Things that could make them beneficial could be:
*If a pink horror dies, he is replaced by 2 blue horrors on the pitch which could take your team over 11 players
*Only blue horrors have right stuff
*If you keep them on the roster, it may take you over 16 rostered players
*Rostered blue horrors do not cost TV
I think for TT, the first idea works quite well and then at the end of the game the player/players is/are removed.
I think with the 3 ideas above (Mutations, Deamons, Horrors) you have masses of scope to make a roster that is very individual and would fit a niche
Spawn
A Chaos Spawn is just about the most stupid creature (or creatures depending on it's make up) in the Warhammer universe. Most definitely one would not come with Bone Head, but would have to have Really Stupid.
A Chaos Spawn has totally lost it's mind and form, and is a mindless...or insane collection of body parts. A curse if you will from the chosen god.
GW changed the law on Chaos Spawn a little around 15 years ago. The old Daemonic steeds were removed and their names became an upgraded Chaos Spawn. One of these being the Beast of Nurgle. So a BoN is an upgraded Chaos Spawn.........which makes it a little troublesome adding Chaos Spawn.
Though if any roster should have something as mutated as a Spawn it should be Tzeentch.
However GW did give us specialist information on the different Spawns:
Beasts of Nurgle drip with slime that burns the ground, their lashing tentacles and rasping tongues infecting their foes with a single caress.
Spawn with long serpentine bodies and many writhing sinuous tentacles are known as Fiends of slaanesh, such creatures race across the battlefield.
The Bloodbeasts of Khorne are masses of muscle and tendon, their many snapping jaws, razor sharp claws and beweaponed tails disembowel with a single sweep.
Most bizarre of all are the Firewyrms of Tzeentch, constantly rippling and shifting with the power of change. Their skin blisters into eyes, maws open on distended limbs, flames lick from screaming orifices and gout out in blasts of purple, red and blue; incinerating those close by.
How this skill manifests is a magic flame/ attck.........Here you could look at the FUMBBL Stunty Leeg Wyrm. Who is a strange beast with AG4 and no hands and throws bombs. A total loon of a player.
So where you'd go with one of these isn't really clear.............but there is a lot of scope.
One roster idea I did for Secret League was on the chosen of chaos. Based on a novel/book idea that chaos warriors that are chosen become daemon princes and those that aren't, cursed to become spawn.
Chaotic Humans
I think Chaotic Humans have become a bit of a mess.
Why does there have to be so many different names? (or potentially). Why can't we keep it simple? I really don't understand the naming policy for the humans in the Khorne Roster. Pit Fighters.......why? They are Marauders like those of the pact team but with a Khorne trait (frenzy). Why not just keep them Marauders or Khorne Marauders?
I'd like to simplify humans in to 3 categories, then give them specialist traits depending on their god
Warriors
5439
Marauders
6338
Cultists/Thugs
6337
This way, you have 3 grades of humans, depending on which fluff (or balancer) you need.
Weaker positionals? Go Marauders. Powerful positionals? Go Cultists/Thugs.
The other problem with Chaos Humans right now is the access. Now, I can see why Pact humans go the access they did. The roster was a spin-off from the old Chaos Allstar roster. And instead of giving them the positionals the Allstars got, they got one standard lino with the access of the allstars (blitzers, mutants and throwers). So this works pretty well.
However the Allstars are something special. Why should other Chaotic Human Linos get weird access. Khorne Human Linos without S access of Pact, but with P access....just highlights why Khorne as a roster idea was so bad under LRB6.
Access for Chaotic Humans needs to be more regimental. Khorne as killers should have S access. Tzeentch as magical mutants should have M access. i.e. better reasoning for the access the players get. And better uniformity with the other Chaotic rosters. A new idea, must fit in with the existing ideas....or the existing ideas tidied up (no reason that Khorne linos don't have P access removed and replaced by S access now that CPOMB has had the 2 brick treatment).
Anyway......I hope I have given you something to think about.