Tie forest generation to rain map rather than a separate density map.

[Retsof90]

Pretty much what it says on the tin.  Seems unrealistic to have shrubland in a high rain area.  I would think tying tree density to the rain map would make more sense.

[Big Shasta]

I recently decided to build in one of these spots... gotta say I was hyped for world gen changes but these in the 18 update really weren't working right (or at all, even, couldn't see any difference on new maps) ... I ended up having to manually set the rain for the area, and it's still buggy half raining non stop.

[Maelstrom]

Soil fertility would also have an impact.  Low fertility soil would not be able to support much plant life.

[Davis]

I think the game needs a bit of a pass at a few areas in game. 

1. I agree with OP, rainfall should determine the forest density / size. 

2. I think this should also tie in to nutrient levels in soil. I am uneducated in this area, but just using logical thinking, more rain in an area means more fertile growing mediums? 

3. Hunger rate settings, day length in IRL time needs to be tied together. Currently if you increase your days to be double the IRL time, it doesn't effect hunger rate, so you need to cut it by 50% to have the same ratio as the standard settings, this doesn't make sense to me personally, it should scale with the IRL day length.