I think Pocus meant that the fastest army gets the choose an battlefield appropriate to the province, not any possible terrain. If that was the case I would stock up on Triton Leaders with amulets of the fish and continually choose "Underwater Plateu" for all landbound conflicts.
I had one idea to make subdivisions of the battlefield were each square held a bonus or a penalty. Would allowed for terrain altering spells and abilities. But I figure that medieval battles were hardly that spontaneous. Perhaps a "Prefered Battlefield" setting could be worked into a battle with the speed of the army, experience of the commanders, stealthiness and random luck worked into getting your choice. A number of Satyrs ambushing a force of heavy cavalery as they travel along a forestroad means that the ordinarily outclassed Satyrs get a huge edge since cavalery can not move into "Dense Vegetation" squares.
Dominions V material perhaps but Fantasy General style battlesquares would be a real thrill. But it makes for a very different game I guess. Still would be neat to be able to draw up a quick battleplan, on a topdown representation of the battlefield, before the battle starts (or just press "Use preselected formation" and hope for the best). Since you don't know your opponents battleplan until the battle actually starts you can get quite a suprise. No unit can be deployed more than one full move from starting position. Flying ability is not considered in deployment.
Perhaps the simplest way to get more usage of light infantery and cavalery is to simply make the battlefield, and deployment screen, larger than it is now. Since we can speed up battles now it's more of an option. Bigger field means higher AP has a real impact since you can outmanouver your enemy.