I just finished my first version of Chaos Space Marines. It went pretty quickly because chaos marines are a lot like loyalist marines, just with less regimented tactics and more rippy melee abilities.
The first change I made was to replace the Tactical Marines "rapid fire" ability with "furious charge". I also added Autocannons to their tactical weapon options (and removed Grav). The final step with them was to add an Icon Bearer option. It was a tough call as to what direction to take the Icon Bearer, but in the end I decided the most flavourful thing the Icon Bearers have done in the past is assist casting in one way or another, so I just went with a simple casting buff. I went simple because I view the spatial constraint of my warscrolls as a sort of complexity constraint. Since the rest of the warscroll was pretty much full of stuff that needed to be there, I didn't have a lot of room to do something really fancy with the Icon Bearer. However, I think that a casting buff channels the fluff and past effects fairly well.
I'd like to share a couple of other units I thought were especially fun or interesting to make. Raptors were neat because I got to roll together two units (Raptors and the newer Warp Talons). I've also been reading the fluff paragraphs about units as much or more than the rules segments to get inspiration for what they should do on the battlefield beyond just recreating what they did in the past. Raptors have (in the past at least) the Daemonkin special rule, and some fluff about how they are so mutated and possessed that they are basically daemons now. So I gave them the Daemon keyword, and a Daemonic Visage ability to highlight that part of their fluff.
Chaos Bikers were a bit of a challenge. Chaos doesn't get attack bikes, so I was losing a chunk of hitting power from the Bike Marines warscroll. I wanted to give them something cool, but I've been steering away from impact hits because they seem REALLY GOOD. Reading some Chaos Biker fluff, I was inspired by the description of them as Dark Harbingers, the forerunners of the Chaos Legions. My Dark Harbingers ability may be a bit much, but I thought some variant of the old Sweeping Advance would be a pretty awesome and flavourful thing for them to be up to.
Lastly, I'd like to show off the Chaos Rhino. While the transport ability on the normal Rhino allows all Imperium models to hop in, I restricted the Chaos Rhino to Chaos Astartes models so that we wouldn't get Daemonettes and Marauders rolling up in Rhinos (it just didn't seem appropriate). The second shift is that the normal Rhino had an ability that let it have a chance to repair itself. I wanted to do something similar for the Chaos Rhino, but in a more hellish bent. So I came to the Possessed Hull ability.
I actually managed to get Possessed Hull, Dozer Blade, and Extra Armour all into various vehicle warscrolls so that if your favorite Predator is modelled with a really cool Possessed Hull, you can easily port the rules over. Because of the constraints of a warscroll, I didn't want to have every option listed on every scroll, but I figure an inventive player with a friendly opponent (they're already letting you use warscrolls some guy on the internet made up!) can recreate their favorite vehicle without too much trouble.