Terrain has played crucial roles in nearly every conflict in human history, with the Great War showcasing this in a variety of ways since it encompassed many theaters, and thus many different areas with differing terrain. Fighting on the Italian Front was very different than that of the Eastern Front, or the Western Front, or in the Middle East. Terrain was key in shaping tactics in these areas. It would be a great improvement if Supremacy 1914 included this feature in the game.
Although visual terrain features on the map could be added, I will mainly focus on the gameplay implications of this feature.
Each province tile could have an assigned terrain value, such as dense forests, open plains, hills, mountains, etc. Each terrain type would have different effects on combat. For example, open plains would give a positive combat modifier for cavalry, armored cars, and tanks, while mountainous terrain would give a negative modifier to those units. Terrain could also give a general positive or negative modifier to attack or defense; it's easier to defend in the mountains than it is to attack into them.
In terms of game strategy, terrain would make players consider the value of certain provinces. For example, one might face the decision of having to reinforce a sparsely-manned mountain province to create a chokepoint against the enemy, or fall back to regroup while allowing the enemy to occupy it.
Likewise, it would also play a role in army composition. Playing as Germany, do you decide to invest in artillery and infantry to fight in the forested western regions, or dedicate your industry to tank and armored car production for use in the open east?
This could be extended onto the sea regions of the map as well, although it'd have to be handled much differently.