Well considering most likely less than 20% of players made it Arx, I guess making a mod for it won't worth that much effort.

Really? I played through the game at least 5 times, probably more (I've lost count)

Almost no mods are made that actually are part of the campaign not easy to do in this unlike neverwinter or Elder Scrolls or what have you.
(exception being npcs that sell and rare rare times have some talks along with selling mod class abilities)

Think I only ever seen like 3 mods that add anything in campaign mode like 1 or 2 added a very few npcs to do quick talks to for example and 1 I think added a semi get their mod item quest I think.