Mostly, I have always thought of mods as smaller additions to the game, but what do I know anyway? I am about as far away from being as expert as one can get! Some smaller mods:
First and foremost, Third Person Camera mod! I will dance for days on end if that is released! /em Happy, Happy, Happy Dance!
Map mod with important points , resources, etc. listed, for those of us who would just as soon skip the whole 'scavenger hunt' aspect. Also a tiny mini-map that one can keep up in the top corner of the screen.
I hope to see spears/halberds/pikes in the game, but if modding is necessary to get them, so be it! It should make for some interesting fights as person with halberd or spear would have awesome range advantage, but would be in trouble if enemy gets too close. Footwork for the win!
As far as large mods go, Ireland would be awesome. If Roman mods are done, they should absolutely include the Germanic tribes (hopefully as a playable race??). Think Arminius vs. Varus, in the beautiful but terrifying Teutoberg Forest, 9 A.D.!