Probably the Old Ones

Also not much of an agri-world if it's got deserts, frozen glaciers, and swamps(? I forget what the other biome was) on it.

The Mechanicum terraforms agri-worlds into agri-worlds, heh.

Sure it would be awesome if the world developed dynamically

Squat DeathStar.

That is all.

Arkhona is a giant meatball...that's the most logical explanation why everyone is fighting for it...

Oh okay... but only since you asked nicely

I'm thinking that Arkhona is a Sub-Sector capital, which would explain its importance and mix of environments.

Miguel said in the Angry Joe interview that if one race actually controlled Arkhona, they would control the galaxy.

Also, the name has 'Ark' in it. Hint?

You are a genius! It must be an Old Ones ark, some sort of vault of secret ancient weapons!