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Ah a Dark Mechanicus army, sounds good and would be one to see.

If you're looking for army comp to go along with a Dark Mechanicus list, some suggestions:

For elites, max out Dreadnoughts. Chaos Dreads are fun. One of my favorite units in the list, provided that you give them a safety bubble. One thing I've done is run two with Autocannons and Heavy Flamers up along the flanks of a Land Raider so that their Fire Frenzy can't actually hurt anything.

Land Raiders are obviously helpful. Cheaper than the Imperial variant, too! If you don't take them, I'd suggest to go with Obliterators. Their official fluff is that they are possibly the surviving Dark Mechanicus that originally sided with Horus and found the Obliterator Virus.

It's really up to you. Thematically, it could be really awesome. Just make sure you put the time into it to make it good.

And the land raiders can take the dozer blade.. Imperial envy speaking!

So, rumors abound for the next CSM book, though it's still in the ether for now. So, what would you like to see?

Chaos Lords should be beasts of hellfire and horror. Seriously. These are guys that are so badass that they have slapped reality in the tits and are making a niche as a psychopath's psychopath. What should that mean? Well, they'd obviously be more expensive, but they should have the stats to reflect it. 1on1, they should be able to win against anything reliably except for named characters of particular renown or some monstrous creatures.

What does that mean for smaller lists? Chaos Lieutenants. The calloused and scarred right hand of the Chaos Lord. Lower points, lesser stats.

Bring back daemonic gifts, but balance them. Nothing quite so extensive as the dial-a-hero that you found in the 3.5ed CSM book, but perhaps something akin to Space Wolf sagas. Still enough that you can personalize a leader without having unnecessary options.

Chaos Space Marines should be hardasses. Many have been alive for thousands of years and have been corrupted by time in the warp. What does that mean? Higher base stats and costs. Basic stuff, a +1 here and there depending on mark. Make the marks necessary on purchase. Chaos Undivided get the Ld bonus, Khorne get +1 A, Nurgle get +1 T, Tzeentch get +1 BS, Slaanesh get +1 I.

Cult Marines should take that formula and go a step farther. Make them as current, but...more badass, which means more points heavy. Khorne Berserkers get rending. Noise Marines get bonus defensive grenades on top of the other gifts and keep their access at sonic weapons. Rubric Marines go back to being monstrous juggers with 2 wounds and keep the AP3 rounds, but go up like mad in points. Plague Marines can upgrade to spread the Rot around them and get access at poisoned weapons.

Now, all of this seems rather points heavy. I agree. That's where the Troops choices expand. Cultists, dregs, corrupted guard, etc. Guardsman stats armed with laspistols and ccw, option to swap out to Lasrifles with a few specials able to be brought in (grenade launchers, flamers) and have the option to have an Aspiring Champion attached to keep them in line Commissar style.

Keep their other options wide open. Make their Fast Attacks actually viable to take in large numbers without being hideously expensive. The heavy support is pretty good as is, though I'd like for Defilers to have either more options or better viability in game.

I'd say make Chaos Lords around the same stats as the Grand Master really, and around the same equipment, but change the Force Weapon for a power weapon, and automatically come in terminator armour, but have the option to downscale to Daemon armour (2+ then 5+ Invul), or something like that.

they are the Rebels of this universe, first and foremost...

khornite army is easy to start off with some berzerkers in rhinos to get them into combat fast and safely, plus it has some fun characters to use (Kharn)... just watch out for enemy shooting because you will have to suffer it a lot