2. the terreforming in FfH is also good. we could use the Fellowship of the Leaves ancient forests spreading as a racial benefit for the Woodelves. (posibly also for Highelves, but to a lesser degree.) the Hellterain, like Ploe said, would be perfect for the Chaos deamons (im wondering if we should divide chaos into 3 civs, (beasts of Chaos, Hordes of Chaos and Deamons of Chaos) each could spread varying degrees of 'Hell'.
use this conjsept to allow Skaven to sprea Swamp, Lizardmen to spread jungle, Dwarves to alter hills and mountains to dwarven versions etcetc.
of course, one drawback of this is that it would be almost impossible to have some form of terraforming for every civ, so perhaps it would be better just to have wonders that spread terrain, or religions.
Hey what about the idea of making hills and mountains in Dwarven terrain to become special Dwarven Hills and Dwarven Mountains (Moria comes to mind), which provide boosts in tax and maybe even culture and of course increase the defense modifier.
So If someone captures a Dwarven city they would recieve some kind of a bonus from the Dwarven mountains - this would be intended - as the ones who conquer Fellowship of the leaves still have the ancient forrests. Dwarven "Mines" should not disappear after the Dwarves are conquered (again: Moria

One could give the Dwarven Hills also a food bonus, which would make it fair that they can't use Vitalize without Fellowship of the woods.
Maybe the Dwaren Hills could be spread like hell terrain, and disappear (degenerate to normal Hills) like hell terrain, or after x turns or never.
Maybe one could make an ice age hell terrain version, to build scenarios before the ice age started.
Also Dwaren Hills would be a safety against Hell terrain, because it doesn't make a difference if you dig under hell terrain, ice age terrain or normal terrain (as stated in the Flavour of Kazhad).
Maybe make them available only for the Kazhad because Lichurp never went under hills like the Kazhad did.
or make this available with Arrete, so everyone may do so (if kilmorph available). But I'd prefer to let it stay at the Dwarfs.
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oh and by the way: why can't orcs build farms inside jungles like in all the mods? I think this really needs to be implemented (without the need of using a mod). It works well, it is not overpowered and it's fun.