I would love to see rivers slow enemies, when I first began playing I thought they did (and suffered the consequences of trying to defend against it...) and I would love to see how a slowed march through water (with perhaps heavy units such as samurai unable to cross?) could turn the tide of battle.
I also very much agree with the prospect of adding the special ability for Yari troops. Too often cavalry units have simply rode through my spear line, suffering a few casualties but ultimately bypassing it. I think either cav should simply "bounce off" from a direct attack (meaning the unit scatters) or only the samurai unit should even be able to mount an attack, as only the most well trained horse would willingly charge into a spear, as the Romans found out and used to their advantage.