Not all of Mordor was ash and fire:

The southern part of Mordor called Núrn was slightly more fertile, and moist enough to carry the inland Sea of Núrnen. Nurn was made somewhat fertile because the ash blown from Mount Doom left its soil nutrient-rich, thus allowing dry-land farming.

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Picture of Núrn

An interesting theory:

One possible theory to Mordor's creation, based upon maps of Middle-Earth in the First Age is that it maybe the dried seabed of the once inland Sea of Helcar

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Nurn inspiration:

Another thing I would implement, are the battlefields of world war 1 that inspired tolkien to create a mordor. These battefields would be good inspiration for the many ditches, holes and crevices that frodo & sam discover on their way to mount doom.
Of course this will not be a complete copy, but we can derive elements of world war 1 trenches to implement into the builds.

Will try to find some pictures later.