True, maybe maps with those perma trenches but makeshift really shitty ones that give just basic cover and can be built within maybe a couple minute. Should have gone a little more in depth with question tho. The animation could be a makeshift one that looks like earthworks on top of another earthwork and mabye sandbags on top. It is one sided and as I said already shitty. It can still take a bullet or bullets. It at least means that events are not totally biased towards the defenders.
Edit: I believe Sapper lines would be good but only for traveling from arty positions to the perma trenches and not to the makeshift trenches I proposed. I'm not a modder but still believe it would be easy enough (since they have made badass weapons) to just replace the chavaux de fries's animation and such to the design like this except with only the side showing up and it being higher up at least to waist level which allows you to kneel behind cover.
The terrain can't be edited during game play(to my knowledge, I've never seen any mod manage that) so player placed trenches would be on top of the ground. Basically, they would look completely awful.
Edit: I feel like I'm misunderstanding your post, but that may just be because it's late. So I'm probably blabbing silly things right now. xD Anyways, to the silliness. I don't know what an animation of a "earthwork on top of another earthwork" looks like. I don't see what some mounds of dirt would need an animation for. Still, I find a lack of need for these player constructed trenches. As I said, they would be above ground and look really stupid. Also, you already have normal NW mounds and sandbags to serve the purpose of cover in no man's land, and look exactly the same as what you're describing. Again, I'm probably misreading, and if you have a more detailed explanation I'd love to hear it.