Hey, got a some ideas for you (and might use a few myself
) hope they help:
If you cant face doing a mountain of skulls for khorne, you could try rivers of blood? Maybe with waterfalls and stuff to fit in with the need to build vertically. You could line the banks of the water/blood with skulls to keep it in theme, maybe use warmaster skeletons.
With the nurgle swamp, how about a hanging gardens/swamps of Babylon/nurgle? Multi layered swamp ‘garden’  with floating bits and waterfalls of filth. Could even have the rivers of blood and rivers of filth from khorne and nurgle mixing at a join.
For epic slaanesh terrain why not try basing it on life size body parts, have hands melded into the ground and various sensual body parts etc. - from a models eye view it would just look like ‘terrain’ but looking from above you’d see it all. That or make a massive noise marine mosh pit.
Id also recommend reading the first part of Dante’s divine comedy for inspiration and imagery – for example the grove of suicides where those who committed suicide have had their bodies replaced with trees, which bleed blood not sap when their twigs are snapped. I seem to remember there’s also a  bowage where a great daemon eternally cleaves the dammed in two, after which they slowly heal as they crawl through filth, only to be cut in half again - some strong khorne/nurlge imagery there. Dante’s portrayal of hell actually lends itself far more to GW chaos gods than you’d image, since it is not until after him that our veiw of hell just becomes solid flame - Dante is far more imaginative with his punishments.
