The map rewrite has been discussed a fair bit over the years (well before the Villains tangent that turned into the current Premium tangent within a tangent).
The purpose of the Map Rewrite is to allow for a number of improvements as well as to allow for new features that currently aren't reasonable to implement. This list is not exhaustive, but may provide an indication of what's it includes:
- Support for the loading of different Z levels separately. This means Adventure Mode wouldn't have to load all the layers of rock below, and would also mean that excursions from a fortress (as trade/war parties, entry into other dimensions/across portals in the same dimension, etc. would be feasible without slowing things down to a crawl. It's more or less necessary to support player activities in multiple areas concurrently.
- Support for multiple maps (such as the normal world and another dimension).
- Support for portals connecting non adjacent parts of the map to each other, both on the same map and to different maps. This can also support maps that wrap around (moving over the edge to the west causes you to enter the east edge of the map, resulting in a cylindrical world, or even wrap around north to south as well as east to west, resulting in a shape that doesn't exist in the real world). It will NOT support spherical worlds, because of the mess caused by trying to turn a sphere into a grid with equally spaced and sized tiles (World maps create enormous distortions close to the poles, which doesn't matter to the real world people as nobody lives there).
- Support for moving "terrain", such as boats and lifts, floating/flying islands, etc.
- Support for sphere influence on terrain/flora/fauna, which probably also supports magic influence over the same things.
- Other support for future expansion. The light/heavy aquifer implementation is a little odd because the data structures have run out of bits to store more properties in. Note that a lot of things the rewrite will support won't actually be implemented until much later, in later arcs. Portals, when they are implemented, will start with one way portals where stuff just appear on the player side, with no access to or view of the other side, for instance.
The Map Rewrite is the first part of the Big Wait, with the second part being the initial implementation of Myth & Magic, which will make use of some of the things permitted by the Map Rewrite. When asked whether the Big Wait could be split up into a Map Rewrite and a Myth & Magic phase a number of years ago, Toady didn't see any point in it. The Premium tangent may change the circumstances, but if it does, it probably won't impact anything until it actually has been experienced (the Premium version decision has already led to the decision to have parallel support/bug fix/minor content development of the released version with the development of the new version, and the longer the Long Wait progresses the more of the new current version content will have to be thrown away and replaced with corresponding content in the next version, resulting in it being implemented twice. At some point it might be considered less work to switch to the Map Rewritten baseline. However, that's purely my speculation).
The Long Wait will be long. An optimistic guess would be 3 years, while pessimistic ones can be considerably longer.
It can be noted that we're still quite a long way from the start of the Long Wait, it's possible it's still further away now than it was when the Premium tangent was embarked on, as there's a lot to be done before it can start (again, the list is probably not complete, and may contain errors as well):
- The Premium release will have to be released (obviously).
- The Premium release bug fixing and immediate problem fixing phase taken care of (this may well take longer than usual, given that the Premium release contains the commercial Premium version, which may require considerably more support and polish than releases have received in the past).
- Implementation of things cut from the release that aren't intended to be cut from the game (Adventure Mode and the Classic versions have been mentioned as candidates for release content cutting of this type).
- Finishing the Villains tangent.
- Improved sieges/army stuff.
- I think there's at least one more point I've forgotten...