I've been playing on the PVP server and the Trench Gun is quite possibly the most fun weapon I've ever used on any minecraft server. Imagine my dismay when I find out it's not in Nations. It's a perfect counter to melee weapons but shut down by long range guns and vehicles.
The trench gun occupies a niche that isn't filled by any other weapon currently. Right now the only way to fight melee up close is to use melee yourself, which makes for a very limited meta. Anyone who is more skilled with guns than melee is completely shut down in CQC.
Some think the trench gun is too OP with how it counters melee, but this is not the case. The real issue is that most melee users fight completely brainlessly, because it's what the current gun roster encourages. Melee users don't even really have to think currently, as I've seen on Saigon. Melee users just rush at you in a straight line and get confused when they're pumped full of pellets.
You can't just rush at someone with a shotgunner, you have to play more crafty. Lure them around corners and into buildings, keep yourself out of their line of sight until they get too close, and then start jumping around them and hitting them with your melee weapon. The closer you are to someone, the faster you move across their screen. They're gonna be swinging their mouse around, unable to lock onto you.
The most danger a melee user is in against the trench gun is when they're trying to get close. When you just rush them with no thought, the shotgunner can reliably hit 5-6 four damage shots while backpedaling and kill you. The trench gun's damage is the highest up close, but its most useful range is actually right outside of melee range.
Another key thing that balances the trench gun out is that it has a magazine size of only six shots, and a fairly high firerate. When a trench gun user starts unloading into you, you can just splash some pots on the ground and let them drain their magazine, afterwhich they're extremely vulnerable what with the six second reload. That's the best time to kill a trench gun user, or at the very least force them to retreat, at which point you yourself could pull out a garand or a mosin and start taking shots at them.
I'd say that's the main thing that keeps the trench gun from completely deprecating melee weapons. Melee weapons don't have ammo counts to worry about, they can just keep wailing on you. You can't just sit still and throw pots at the ground against a melee user, that's opening yourself up to some nasty crits. The only way for a trench gun user to do the same is to willingly run into your melee range, which puts *them* in danger.
Keeping all of that in mind, the trench gun is far from overpowered, as it's hard countered by long range weapons and vehicles, and while good against melee, it's far from unbeatable.
Currently the only trench guns in Nations are rare drops from the anniversary crate, and they all have curse of vanishing. I don't know if this is a test to see if trench guns should be added fully, but if it is it's a huge misplay. Nobody's gonna actually use a rare weapon with curse of binding in combat, they're gonna hold onto it and hope the value appreciates over time.
The only way to truly test to see if the trench gun should be in nations is to throw it in there. If it's really not healthy for the server (which i doubt), you can simply remove it from crafting and add curse of binding to the existing guns. There's zero reason not to at least try the trench gun out in nations.
Final point, it's just super fun. Shotguns are awesome, and the firerate on the trench gun is unlike any other shotgun i've tried on a minecraft server.