Just guessing, but if the new larger puzzle pois increase generation time, then adding swers would be the same kind of generation hit.
That said, maybe a18 could be the underground update? They could focus on the perlin cave system, underground pois, and large city sewer system generation. I would think you would only really need maybe a dozen pois that are tiled for the entire understreet system, with maybe a few specialty ones like a big cistern with a bunch of sleepers, or a big underground water treatment plant.
That would also give the code monkeys more time to really sort out bandits and the quest systems, but it would give us a new update in much less time that a17 and give us a significant addition to play without a huge development time hit.
An interesting addition might be a collapsed street poi leading down into the sewers, blocking off easy access to some parts of it. Manhole access could be every few city blocks, making it a little harder to explore the entire thing.
If pois can be stacked in rgw, they could even add underground storage rooms with sidewalk access (like they used to use in large cities.)
When I worked in Cleveland, we had a big steel door in the basement of the store I worked in. We learned that the city actually had an underground access tunnel under the sidewalks, which connected to store rooms under each store. They all had steel doors between them separating them. Deliveries would use an elevator in the middle of the sidewalk. (covered by a steel grate when closed)
We actually found the store key to the system, and went in one day after hours. It was ungodly creepy, and we didn't go further than two or three storerooms down because it just was THAT creepy.
I just present the idea because there is a lot that can pretty easily be added underground, and thats one of the things we love about minecraft still... the underground doubles rwg exploration.
And now that we have the gyrocopter, I really want the code repurposed for a parasail to quickly descend from tall skyscrapers. If link can do it in BOTW, we should be able to do it as well in 7 Days.
(explosive crossbow bolt elevation boosts could even be added to keep from descending onto a zombie bear or ferals!)