Looking here:
http://darkstars.co.uk/2007/prometheus-grounds-ta/
Or more specifically, here:
http://itchstudios.com/psg/ta/ta.htm
We see he doesn't just have unit concept art, he has a storyboard of a reimagining of TA:

He has 3 images like the above complete with scene descriptions of what should happen.
I suggest we take this as a starting point, but change the faction names, and refer to the previous war as 'the great war' and leave the details vague ( useful for any campaigns we may try to build ).
Niklas has always been happy to let us use his artwork in the past, and if asked I'm sure he'd let us use these, possibly even make us some new artwork if we're super nice.