Everyone knows already that Warhammer 40000 8th edition Starter will bring us Death Guard versus Ultramarines. So, Dark Vengeance will (finally!) be out of date. But there's a big problem - that starter set was a perfect way to get many Chaos Cultists at once. Otherwise, you have to buy 5-men boxes with fixed poses and different wargear. The big box of 20 men is just 4 of those 5-men sets. So, let's look at some ideas of how to convert Chaos Cultists - soon it will be a very useful skill.
However, this doesn't solve a problem with wargear. You'll still have 2/3 different cultists from one set. Also, you can't make a champion this way.
You can diversify your squad with other models - it won't look solid, but the WYSIWYG will be OK.
Revenge Games Workshop and use miniatures from other wargames. There are a lot of armed men in bad armor in almost every game system.
Some tried to use Bloodreavers from Age of Sigmar as cultists, but it's not the best idea. Yes, the miniatures are great and look like cultists. But they are too big. The cultist is not likely to be as big as a spacemarine.
A great alternative is renegade models from Forge World. The have vide model range and different wargear. And they look perfect.
The only problem is that they are equipped too well for usual cultist scum. Maybe it won't correlate with your vision of the army.
Use Astra Militarum models. Cultists are often guardsmen-renegades. Work with their faces and the whole bunch of cultists is ready.
Flagellants from Fantasy Battle are one of the best alternatives if you don't like armored cadians models. Minimum changes and your cultists will look much better than their standard version.
Besides, there are many ugly and damaged (both in a good way) heads which you may use for your Chaos army.
The last way - try something else! Only you decide how your cultists should look. Use any sets to get something unique and cool!