| WWHsmackdown |
It's a blasted wasteland full of magical radiation so potent that traditional spellcasting goes wonky. I'd play up the ambient energy in the environment by letting the players take fort tests to avoid being constantly sickened in an environment that is quite possibly toxic and mutative. The weather, terrain, and laws of physics are also probably subject to sudden drastic change if residual magical fallout pools in some area at any given time. Also, don't forget about the mutants and monstrosities roaming about, be they fauna that have changed due to energies, inanimate formations that have decided to stop being so, or fleshwraught weapons of war and golems left from Nex and Gebs conflict. If you wanna get super technical you could also have your casters roll on the wellspring archetype table every now and then to simulate the tumultuous proclivities of magic in the area (if you let their magic function at all in the mana wastes). Think Mad Max mixed with Alice in Wonderland.
| Tender Tendrils |
Depending on the parties level, there are a bunch of environmental and magical hazards you can use - stuff like quicksand, treacherous scree, steam vents, yellow mold, planar rifts, etc. (various books)
The gamemastery guide also has the planar traits - thought normally traits for different planes, these traits are all weird environmental stuff that can be pretty good to have apply to random patches of the mana wastes. Stuff like differing amounts of gravity, different passage of time, different types of magic being enhanced and impeded, etc. (GMG page 136)