Although rumors suggest the newest Daemonkin book will give Tzeench some love, a Nurgle themed codex can't be too far behind, especially considering the crazy fast releases from G-Dub these days. Just brainstorming some ideas that would make it a fun release...
1. Release the new plastic Great Unclean One - this is an easy one, we all know he's coming. Should be a stellar kit.
2. Plague Zombies - They are already a choice in the Chaos Marine book, so it makes sense to release a kit for them. Considering that the old zombie kit has given us so much mileage over the years, this is a no brainer. Raising zombies over the battlefield would be super fun and characterful too. Here's an idea; either make your Nurgle sorcerers able to raise units of them on their own, or you get to replace a unit that was wiped out with a random number of them.
3. Traitor Guard - This is a bit of a reach of course, but wouldn't it be great to get some god specific Traitor Guard in these books? It would give it a true "Lost and the Dammed" feel for sure. You already have Cultist miniatures; why not expand them list-wise to be able to be true Traitor Guard troops? By making Imperial Guard not being allowed to ally with Chaos Marines or Daemons anymore GW effectively cut off many of us not being able to use our converted Traitor Guard, so hey, add them back in!
4. Death Guard - I want new models for these guys. The little Marine scale models of the past just don't do it for a lot of us now, especially after reading some of the fluff and novels about how Marines are walking gods. Being not that much taller than a Guardsmen needs to be changed. A bigger kit for them means sicker conversions for Nurgle troops! Rules wise they just need to be hard mothers to take down, I'd be happy with that.
5. Epidemius - This guy needs a new kit, in plastic, so we can convert up some palanquin riding Champions of Filth!