As you know by now I love Forgeworld models and the Death Guard upgrade is awesome. The only drawback is that you do not get backpacks, legs and arms and the Chaos Space Marine plastics can look quite bare compared to the forgeworld parts so you end up with a model with highly detail parts and bare parts. There are several methods you can do to make the plastic parts more interesting and Nurgle-esc.
Guitar String
Guitar string can be used to add random pipes sticking out from your marines. They can be tricky to bend so I have not used a lot. I plan to paint the tubing on my Plague Marines to look like maggots.
Green Stuff
You can go nuts with green stuff for Plague Marines. You can create rotting armour, boils and even guts without even needing to be a green stuff expert. I can barely fill gaps with the stuff but when it comes to Nurgle I can just stretch it and poke it until I get something that looks very effective.
Drilling and Carving
This method I have used more than the other two as it is quicker and easier and still gives great results. Just by drilling holes and carving away at the armour with a hobby knife gives you nice holes in the armour that represent Nurgles rot or acid corroding the armour away. The same method can also be used for bullet holes and battle damage.
I hope these simple methods have given you some ideas if you plan to make your own Death Guard Plague Marines using the lovely Forgeworld parts. Below is the full 7 man squad.