ok here's an innovation I put up to someone else a while ago and I think this could definitely use it.
Most trench tdm's suck mainly because players start with nothing as a base and everything is out of place. Most of the combat involves walking long distances which gets boring fast, and trying to dig anything is really ad-hoc and doesn't work well since its easy to die, and the buildings that take a while to create do little in actual "defense". Tunnels are extremely easily taken over or infiltrated and are a tactic that, although would be really innovative and fun, fail to live up to the digger's imaginations.
What I proposed was what you could call is a traditional TDM with Trench digging added in. In the design, each side would have a base (built on baseplate plates so you can't dig up the floor) which would provide cover across a "no-man's land" about 3 64x baseplates wide. In each base, a specific area in the back would be open (uncovered by baseplates) for trenching tunnels. Players, instead of having to build everything - bases, cover, trenches, etc - would have an area for fighting already, and in addition the solid bases would mean that you can eliminate the long walking distances to prevent trench player griefing (at least to a point).
In this design, digging in the no man's land would (as it originally is) be extremely dangerous since players on both sides would be shooting across it. This forces players to instead, use the back area to dig tunnels that lead out into the no man's land, where then they can dig up to create trenches, followed by cover. Players that are smart would also dig defense tunnels that are meant to intercept enemy tunnels and render them unusable by camping them, forcing diggers to be fluid in their ability to adapt tunnels and/or quickly wall off some tunnels and dig new ones, or dig countermeasure tunnels or open caverns where some players can sit inside and/or use walls for cover.
You could also make it a CTF instead, and in this case then players would have to dig and infiltrate the enemy base, trying to navigate around and access the enemy's back "digging area" and using that to get the flag before retreating into the tunnels to head back.
This design is true "trench" warfare since the emphasis would be on making trenches rather than elaborate structures.
Poll