A21's RWG mixer rolled...
I think you mean RWG and not the rwgmixer.xml file, but I'm not sure. I believe you could make an A21-like rwgmixer.xml configuration, but I don't think that gets you want you want... (details follow)
A21's rolled a map with a cabin in between two nearby towns and a trader on the border of each near the cabin. Towns were so close together, it was just a hop, skip and a jump to both. (Towns are generally pretty far apart from one another now, rather than a mixture of near and far.)
I don't understand this. The cabin would be a Wilderness POI. Those would still get placed randomly between towns. The rwgmixer.xml file wouldn't have controlled that and the current RWG code still does that.
By towns I think you mean settlements (Cities, Towns, CountryTowns, WesternTown) and I think you're right in that RWG didn't divide the map into a grid of tiles when planning the map's layout, but that's not in rwgmixer.xml either. Note that there are two different meanings to the term "Tiles" when we talk of RWG. One is RWG's planning of the overall map layout and the other is a POI-like entity that describes a specific layout of 150x150 settlement content.
I could be wrong, but I think RWG's current approach to dividing the map up relates to RWG's performance today. That is, you get a map in 1-2 minutes now instead of 5-10. If that's the reason for your observation, then it isn't in rwgmixer.xml either, but in RWG's code instead.
If it was performance related, that's an interesting trade off. TFP ate criticism for slow map generation, so I can understand making changes to improve map generation times. Personally, I don't mind waiting for a good map, but that's just me, though I do enjoy being able to make 5-10 maps for testing in the same amount of time it used to take.
A21's rolled a map with an Eye Candy factory right down the road and around a curve from Jen (when Jen could appear in the Forest biome), Jen's compound touching the edge surrounded by a few farms; and a map with Hugh's compound adjacent to a city's "downtown," but not so close that it was downtown. Hop, skip and a jump from an office complex.
I'm not sure I understand if the placement of whichever factory you mean is the point or the placement of traders is the point. I had to go back and look at my modlet for historical context. In A21 there were many gateway tiles possible (some of which can have a trader) and if I recall correctly one of the traders had a small chance of being placed in the Industrial district. I thought it was Bob, but it could have been Hugh. I think you'll need some alternative trader POIs and maybe some supporting city Tiles for this, then a change to rwgmixer.xml. For instance, the current Hugh trader will only land on a Gateway Tile.
The factory thing I want to think is still possible today, but there's not a lot of detail in your description.
A21's rolled maps with interesting views from hills and mountaintops. Fairly begged for hills and mountaintops to be used for a base of operations with runs into a nearby town. Haven't seen an interesting view since.
I don't know how rwgmixer.xml will help or hurt this. I don't know how the placement of hills has changed, but it isn't in rwgmixer.xml. I thought that was all handled by stamps.
Etc. and so on. You'll not see any such maps now. Maps have been made as grid-like as towns and cities and, therefore, variety-less. No more nice surprises. Also, I don't buy the argument that only traders have the sense to stay away from the cities and only the player dumb enough to set up permanent residence in them. In A21, traders were sometimes close to, but not in cities and towns as well as a good distance from them.
In short, A21's produced more natural-looking maps.
I can't debate your opinions or observations and I'm not sure I understand what you mean.
For instance, I disagree about "suprises" based on a feature newer than A21 with Gateway Tile placement. Those tiles are rolled into road connections now and also the only place where traders can land. You get some lovely little t-junction and intersections out in the wild now where roads meet that you didn't get in A21.
On the other hand, trader placement is tied to Gateway Tile placement, which makes trader placement much more predictable. They will always be on the edge of town or on one of those road junctions out in the wild. To break that pattern going back to A21 won't help. You have to change the traders and probably make more settlement Tile variants to support them, which is possible, then likely some related configuration in the rwgmixer.xml file would be needed.
I can perhaps agree that there could be a bit more variety in the settlement Tile layouts from A21, but not in POI selection which has gotten bigger as the versions have progressed. RWG was probably more "daring" with it's selection of Tiles; that's not rwgmixer.xml though. That is, the RWG code into 2.5 had a flaw in that it strongly prefered intersection Tiles to the exclusion of others. That is said to be fixed in 2.6.