Fixed terrain is a pretty silly decision. It will now make participants in wars rush the closest mountain province and bait there.
Rather than throwing out the entire percentage system, I would have liked to see it replaced with a sort of tiering system based on old percentages that ties into general maneuver to determine the terrain that a fight would take place on. The old "dominant terrain" would be the new default for all battles, but depending on the maneuver difference between the attacking and the defending generals, the terrain would shift to being more favourable for that side.
As a very rough example, say a province used to be 40% Hills, 30% Plains and 20% Mountains. In a tiered system, all battles would take place in Hills; but if the attacker had a sufficient maneuver advantage over the defender (in this case, let's say +2), the battle would instead happen in Plains. Similarly, if the defender had say a +3 maneuver advantage, it would happen in Mountains.