I think that if you are going to do lava bases for one part of the army you should do it for the whole thing. One of the problems with the Daemon line, as I think I may have commented on in a different thread of yours, is difficulty in visual cohesion. If you base them the same, with little differences bewteen the different gods, like sculpting writhing faces into the rocks for Tzeentch, adding some pestilent pools of ich along side the lava for Nurgles, and though I would love to add something for Slaanesh I am lacking creativity right now, it would help give the army a visually consistent look.
I am actually glad that I stumbled onto this thread because I have been suffering the same round/squard dillemna with my own Daemon army. I think that I will also end up going with the square bases so I can play in both systems as well. Good thread.
Yeah I'm in the same boat, I'm incredibly tempted to go mono, but I know the minute that I settle on one theme i.e. Khorne, I'll immediately want Slaanesh, Nurgle or Tzeentch as well.
My thinking has led me to the opposite conclusion of yours, basically .... screw cohesion. By embracing the differences of the Chaos Gods, I think you will have a unique and cool looking army.
My fluff thinking (which is always my number 1 priority) is that a horde of all four Chaos Gods ripping through into real space is the most epically awesome sight to see, in the most disturbing way.
You should be assaulted with loads of different sights, sound and COLOURS, meaning that army cohesion is actually what you don't want.
This is why I'm thinking the way to go forward base wise is to highlight the differences as follows:
Khorne - Black, Lava, Skulls, Blood
Nurgle - Garden of pestilence type imagery, so use dark green grass flock, but in putrid colouring, with as you suggested maybe even a few Cesspools or weird looking flowers.
Slaanesh and Tzeentch are where I'm having the most problems.
For Slaanesh I'm literally of doing the standard green grass/goblin green bases but with purple lines cracked through it to indicate the corruption of beauty Slaanesh represents.
For Tzeentch I haven't a clue.
As always though I think my ideas are waaayyy to difficult for my skill level as a painter, so it's one of those sucky situations were you can imagine the
army but can't actually complete it the way you want. Not to mention budget concerns and other projects like a tactica, painting guide, 40k story, fluff guide, and a little thing called a PhD.
Looks like I'm going to be a busy bee this year lol.