It would probably only make things worse.
If they put Khaine back together, Khorne and Slaanesh would immediately start fighting over him again. Who's to say they'd luck out and only have him split up this time, perhaps he'd just be killed.
Also, Khaine hates the Eldar, or more specifically, he loves murdering Eldar. Seriously, it's like his whole bag. Sure, he killed their enemies sometimes too, but mostly he murdered them, their heroes, and their gods. You'd have to be in a pretty
Godzilla Threshold scenario to even think about doing this.