Themes: flying, shooty, armor-penetrating. A successful result will look like it does what it’s supposed to do in-game.
This is the hard one. Tzeentch units will, I think, involve the most actual conversion… and the most discipline. Everything I’ve brainstormed eventually transforms into something very Skaveny: dangerous looking, but madcap and zany. That’s sort of thing is great for Skaven, but not so much Daemons. Or, at the very least, not the sort of Daemons I done so far. So, that’s the trap to avoid.
Horrors will likely see some table-time at some point. That I run an army with next to no shooting is kind of crazy. What I’m likely to do here is up in the air.
Flamers, I have a clear vision for. Bloodletter base, with the horns that sprout from the head shaved down (so it looks less dangerous than the Daemonettes and Bloodletters and brainier). The Hellblade forearm is replaced with some rigid plastic tubing and capped with one of the heads from the Chaos vehicle sprue (because it shoots things). Wings from the Gargoyle sprue (because it’s a jump troop).