So basically what you are suggesting is to team up the battles in Legion Arena with the campaign map of Legion II. So that is what a whole bunh of people want is the ability to play the battles in Legion II.
I'd like to see the campaign map be not too large. The LA engine works best at the army size scale, so it would need to be a campaign of that scale rather than a continental or empire building scale.
I'd love to see Slitherine take Caesar's Gallic Wars, integrate some of that great material and imagery (a la the History Channel expertise) and build the campaign in such a way that it focuses on one of Caesar's subordinates. The player would report to Julius himself, but be sent out to subdue a tribe, secure a border, or raid a region. These campaign maps would be of limited scale, but give the player the chance to manuever and pick the terrain on which to fight (or at least try to). Intelligence of the enemies size, composition, and position, weather, terrain, speed of movement, and fatigue, day/night could all play a part.
To me, keeping things to the scale of an individual commander working in the terrain is the next step for the LA engine. And it would be something that to my knowledge doesn't already exist. Aren't there enough empire building games already?
Once they had that model down, then it could be onto to play one of Hannibal's sub-commanders or one of Alexander's officers.