Well it depends on where your hobby strengths lie and what style of fallen you prefer.
With painting you could try to paint blood splatters on them, add much battledamage and chaos runes and iconography - glowing or normal ones.
Kitbashingwise you could add chaos bitz from different chaos sets like possessed or just basic marines, change their heads or add at least spikes and such to them. Maybe swap out their weapons to chaos bolters.
Also you could add small modeling chains to them or add trophies like skulls, helmets and such.
With sculpting you could add typical chaos decorative trimming and chaos details like the 8 pointed star or at least part of it.
I turned my
DV models over to slaanesh with mostly sculpting techniques. But be warned i have a decent experience doing that and it is probably the most time consuming way to convert them to chaos.
Not all pictures show the finished conversions, some are still
wip shots but it should show you what i did with mine.