I say this is really great idea, but also very hard to do in opf. I encourage you & ur team to proove me wrong though.
Without thinking about too much details, these are the things that come to my mind for now:
1.) Sword fighting
should be more than just widly clicking away at ur enemy. Although i can't think of a way to integrate a good attack/parade system in opf. Hope you can figure that out. Maybe don't try to get it that realistic, that could be impossible. 1-3 predefined moves (attack/parade & move automaticly) could work.
2.) Useable, animated horses
would be awesome of course, also hard to do / to code (i guess). If i were you i'd stay away from developing opf horses until you got a 1st prototype of your mod where all weapons do function well. Big battles on foot can be very exciting, too.
3.) Construct castles with obstacle sections
Castels should consist of single walls, corners, doors, gates, towers etc. That way big weapons (catapults) can destroy only 1 side of a castle to let the foot soldiers in.
4.) Encourage strategic economy
(ok, not that important at first) Would be cool to have some sort of economy / resources that 'drive' those armies. I mean, the tanks and soldiers of Opf have their fuel, endurance and ammo with them. What do foot soldiers need, except of endurance ? Water and food and treasures maybe. Wouldn't it be great to slowly win a big castle battle by disrupting the enemies support on water and food ?
Change ammo boxes into bread boxes, or treasure crates. Replace fuel stations with water sources like fountains etc
that's it from me, hope you get something going