I've been playing Chaos Space Marines since shortly after 3rd edition began, and in all that time (and three codexes), I've never seen fit to include Possessed Chaos Space Marines in a single army I've ever run. It's bad enough that they've tended to be overpriced and frequently a real gamble. But, even worse, they're a unit looking for a purpose. Why would I bring them at all?
If I want close combat specialists in power armor, the obvious choice is Berzerkers. Berzerkers are (or can be) Troops choices, which is huge in 5th edition. And they're quite good at tearing stuff up in close combat - better than ever, now. I could also take Chosen, which can select numerous CCW upgrades while outflanking or infiltrating. If I want to move up a bit from power armor, there's terminators. Just a few points more than Possessed, terminators come with stock power weapons, 2+/5++, and numerous options.
It seems to me that most ways to make possessed worthwhile would also make one or more of they're near-equivalents obsolete in comparison.
So, what can possessed have that Berzerkers, Chosen, and Terminators lack? For all the disappointment at rolling it, Fleet is unique, in the Chaos Codex, to Possessed, and right now so is Rending. At its simplest, replacing the random table with those two abilities would make for somewhat faster and better against armor Possessed that aren't quite the same as the other close combat specialists, while perhaps worth their points cost and not over the top.