This is excellent news and will add great replay value for the game!  
Here are some features the community would be interesting in modding: 
1)  New Races/Nations:  And the related details(spells, technologies, items, abilities, units, buildings, etc.,)
2)  New Terrain Types:  examples= Lava, Snow, Caverns, Tar Pits, etc., 
3)  New World Map Structures: examples= Gates, Mazes, Altars, Graveyards, etc., 
4)  New Channeller Shapes/Designs: examples= Saurons immobile flaming eye, Medusa(Gorgon), etc.,
5)  New Battlefield Skills for Units/Champions/Channellers: examples= Turn_to_Stone, KnockBack, Possess, Spray_Web, etc., 
6)  New Abilities for only Champions&Channellers: examples= Tunneling, Alchemy, Stealth, Assassinate, Tracking, Hibernate, etc.,
7)  Adding new game elements: examples= Merchants Guild(Another Trading Method), Arena Death Match (Each_Player_sends_1_Champion_to_Fight_for_the_Prize(s)),  Traveling Weather randomly moving on the World Map, Natural Disasters(Plague), Mercenary Boards(Temp_Hire_Mercenaries), etc., etc., .
8)  New AI Personalities:  Custom AI Scripting allowing the creation of unique interests, personalities and behaviors for the AI opponent.
 
Hope this helps.    