I think a molten rock and lava terrain would actually be very cool for age of fantasy map makers. Molten rock could be a simple broken rock texture, blackened and with red hot seams in it. Most flesh and blood units that end their move on molten rock suffer some damage, wood units suffer catching on fire, but you can run over it without harm.
Lava is just impassible, but looks cool, especially for some campaign maps. I will whip up some images today.
