If I was a decent modder I would make it that cavalry were a very fast moving unit, but I'd make it that their speed suffers in areas with high shelling. So the more persistent shell craters in a battlefield, the slower they move. I'd also make them extra succeptible to barbed wire as it was more effective against horses than footmen (though absolutely effective against foot men too.)
.
Second, I would make it that when ordered to attack a trench, they would travel to about grenade range, then dismount, and the horses would do a retreat animation while the light horsemen now on foot travel whats left of the way to the trenches.
.
I think I would also make it that a horse mounted unit will suffer heavier moral penalties while mounted, but would have higher moral resistance when dismounted. This is to represent the horses being more easily frightened than the rider, and that cavalry units had less fear than the regular army and seemed to enjoy charging machinegun positions headon
.
I'd also make them medium sized. Less bodies than a regular infantry company, but a lot more than a trench raider or grenadier unit.
