So before I get to the suggestion, allow me to pretense this with an explanation. Out of every single thing there is to do in TU, for some reason I just cannot stand Zombie Massacre. I’ve played plenty of similar top-down shooters and had a blast, but Zombie Massacre has quite literally put me to sleep before. So that got me wondering what makes it so different, since I like every other thing there is to do in TU, and after a short bit of pondering I realized: any other top-down shooter I’ve played has you working your way through a level in order to reach an objective. In a sense, it’s a strategy that a lot of Zombie Shooters utilize. Left 4 Dead is practically the king of this (albeit not top-down), even playing the same set of maps over and over again is still entertaining because you’re trying to make your way to the next safehouse.
Now, I don’t know what the eventual goal for the gamemode is, or whether it’s just not designed to be anything like this, but I think that restructuring the gamemode to be more based around moving your way towards an objective rather than just staying on the same map day after day. Unfortunately, this of course is probably significantly less feasible, but I do believe that the maps that are currently made could at least be reused and retooled to be like the classic “survive the horde” portions of the L4D maps, and wouldn’t end up much different than the current purpose they serve. I feel like that level of movement would at least create a more engaging experience, because while the structure of abilities and whatnot feels fine, there’s just no real sense of urgency to it all.
If not that, then I think the sort of more arcade-style randomized encounters on a set of predesigned maps a-la Dead Ops Arcade could be adapted. Maybe maintaining the same structure of taking the helicopter in and out of the same area but just adding modifiers and obstacles could spice things up a little. There are plenty of different ways to approach this, each with varying levels of additional effort required. Whether it be as involved as a safehouse to safehouse style structure or just something more arcadey, I think there’s gotta be something to spice up the Zombie Massacre gameplay loop some more. While I definitely don’t think there’s anything wrong with having to stay in the same general location to survive for a while, I feel like that’s just one piece of the pie.
…Of course, this could all be extremely obvious and be what’s already in the works. Also, I do respect that some people just currently enjoy the gamemode as is. But I just think a little more…oomph would really help.
And to be clear, if this IS already in the works, I didn’t just lazily make this post without looking around, it’s just difficult to find much information with how long the game’s been around now. 
Most of what I know is only from looking at the trello, stumbling into information on the forums or other avenues, and general word of mouth from people who have been around a long while. That said, if this is the case, I would greatly appreciate anyone who could point me in the right direction of where to find information on it.