The 4th world is already in progress and as fas as i know it will be a fantasy/dungeon-world.
I think some of you had the idea already to
have a
jungle-world or rainforest theme?
Somewhat similar to desert-world there could be either pyramids of aztecs/inca or far south eastern "angkor vat"-alike sunken temple-ruins.
- i'd prefer the aztec/inca-theme simply because of the block-architecture.
The aztec warriors (undead, skeletons, mummy-replacement) in the ruins could wear some stylish helmets, necklaces or bracelets (golden weapons & breastplates for the bosses/highpriests?

)
There could be special jungle-flora that will also
block the path so no dwarv can pass by, would not need a backwall to grow on but some surface block below and needs as long as a tree to log - depending on used tool. Jungle-flora could be a mixture of tree & bushes, more than 1 block height.
some sort of tiger or jaguar could replace the boars and be more dangerous
ducks could be replacement for the chickens (ducks love eating snails...)
instead of sheep there could be some mouflon-alike creature (Муфлон) as source fo wool- or the lamas from desert could repeat here.
replacement for tornados would be hurricanes/typhoons,
coming with a heavy load of rain and the new effect of
swamp- (mud) instead of lakes/water on surface
a mixture from a few units of water + dirt which can not be mined but have to be dried out. (collect mud with buckets!)
craft a filter-station to gain clean water from mud
surface-dirt can get wet from rain and change to mud (erosion/эро́зия)
swamp/mud does not drain like water if no stone-frontwall below, stays there forever until collected.
if block below swamp/mud gets mined the mud would behave as water does and fill the area below.
dwarves could sink in, get stuck,
slow down-effect depending on mud-depth:
1 depth slows down
2 depth slows down more - with a fully loaded backpack would barely make 1 pixel per second
3, 4 depth makes dwarf stuck (will smaller dwarves drown at 4 depth already?...)
5 and more depth- dwarves will get stuck and drown if no diving helmet
stuck dwarves will need help from other dwarves and the mud has to be collected until the comrade is freed and can move again.
Undead (skeletons etc.) would use bone-bridges to cross swamp-blocks and of course remove jungle-bushes blocking the way with their swords
...not a request, just ideas
