It turns out that 1500 points is a lot more army than I expected.

Sticking with the Daemons (because they're easier to assemble and (crucially) to paint), here's an initial list for 1496 points:
- Supreme Command Detachment (+1 CP)
- 320 - HQ: Great Unclean One (doomsday bell, plague flail, stream of corruption, fleshy abundance, miasma of pestilence, warlord: virulent touch)
- 100 - HQ: Epidemius
- 156 - HQ: Daemon Prince of Chaos (daemonic axe, malefic talons, fleshy abundance)
- Battalion Detachment (+3 CP)
- 70 - HQ: Poxbringer (virulent blessing)
- 75 - HQ: Spoilpox Scrivener (Entropic Knell)
- 70 - Troops: 10 Plaguebearers
- 70 - Troops: 10 Plaguebearers
- 70 - Troops: 10 Plaguebearers
- 70 - Troops: 10 Plaguebearers
- 126 - Troops: 7 Nurglings
- 102 - Elites: 3 Beasts of Nurgle
- 102 - Fast Attack: 3 Plague Drones
- 165 - Fast Attack: 3 Pox Riders of Nurgle
Thoughts on this list:
- The Plaguebearers can be merged into fewer units, but I've split them up due to my analysis here.
- This is the best amount of command points I can get; I'd need 1 more Troops, 2 more Elites, 1 more Fast Attack, and 3 Heavy Support for a Brigade Detachment (+9 CP).
- The other Great Unclean One is armed with a bilesword, and costs 5 points more, which would take me over the 1500 point limit.
- I don't think daemonic icons are worth the points. I'm avoiding the instruments of chaos too. Just going for a straightforward build for what is my first game in decades.
- Having read the various stratagems available to me, I think that saving the 4 CP for re-rolls is the best option.
- Movement will be absolutely critical with this force. There are few ranged attacks, and I need to get the infantry into melee quickly to save on attrition. The Spoilpox Scrivener is critical to this, increasing the move of all Plaguebearer Infantry within 6" by 2" - this includes himself and the Poxbringer, bringing them all up to 7", in line with the GUO. The Pox Riders also have a base 7" move (but can Fly too).
- Epidemius can bring up the rear at 5" move, keeping his tally to (hopefully) good effect.
- The Daemon Prince has the flexibility to support where needed, with a move of 8".
- The Plague Drones zip around at 10" Fly.
- The Nurglings can infiltrate, which is worth doing, using them to try bogging down any long range weapons platform. They won't survive long, but should act as a brief distraction.
- The Beasts only have a move of 6" which is a bit too slow. They may end up being ineffective. We'll see.
- The GUO and Daemon Prince are obvious targets. Giving them both Fleshy Abundance will allow them to heal some of the fire that they will inevitable soak. I'm hoping they will draw fire away from the Plaguebearers and Heralds.
- I'm thinking of ignoring daemonic summoning. It seems a bit of a hassle, and risky in terms of possible failing and causing wounds.