It's quite possible, in the past (in EWOM), we could add factions (or mod them) to have desert/snow/etc terrain as their defaults. However. if I recall correctly (been a while), they made it so that you only get territory bonuses on Kingdom (grass) and Empire (dark grass) terrains, and most of those modded factions got shafted as a result.

Perhaps. The way the fertility/production mechanic works as is, does kinda inhibit this, I suppose.

I'd just like to see something give more visual distinction to kingdoms in particular, seeing as they're all humans, basically. Right now the only thing that really separates them at a glance is hair color.

+1

age of wonders did this, they had snow, and swamp lands extending outward in your zoc, it was really cool looking and useful to tell what faction was where even at a glance. Personally I think they should all be spreading some type of life, which gibes with the general lore and is intuitively a good thing (as opposed to say, spreading perpetual winter), but that doesn't mean the life all needs to look the same or even necessarily earthly life. 

I imagine the Tarth spreading royal hunting forests, or even jungle, all the better for hunting in, which could double as extra protection (pre-roads I guess) due to the movement penalties usually associated with it which they ignore, I'd love to see some of the magic focused ones spreading a type of primate living elemental landscape.