Thanks for your feedback
As mentioned by paulevs , the screenshots on Trello do not show the final result, so a lot of things may change until these features actually become available 
And dont make beside the forest a desert or a polar region, after the forest comes the grassland with some bushes or also called grass steppe, than a dry land with dry water riverbeds and there are just view plants which dont want much sunlight in shady areas below a huge rock and plants which dont need much water like some dry bushes and knobby little trees here and there, and finally the desert.
I want to say, please make bioms in-between the forest area and the polar regions which named tundra.
Basically these transitions were already present in the Java version (i.e. a desert was always surrounded by savannah, polar regions were always surrounded by tundra etc), but unfortunately there was only a limited set of biomes available... we couldn't add new biomes retroactively without breaking existing worlds, so this part was basically never improved after the initial biome release 
However, there will be a different approach in the new version: As mentioned by Avanar , there will be 1 major biome or climate zone per island. For instance, if it's a "dry climate zone", it will only spawn warm/dry biomes (e.g. desert, savannah, steppe etc). This approach also allows us to add a lot more "mini biomes" (e.g. an oasis in a desert) 
Maybe the climate zones should also right in order:
We have been thinking about that, but we're not sure if that would be really a good approach... As Avanar mentioned, there is no real equator due to the world being infinite (in theory^^). We could, however, make sure that cold biomes always spawn north of you, while hotter/warm biomes always spawn south of you (still not accurate, but plausible at first glance) - but that would mean that you only find snowy islands if you keep travelling north (or only deserts when travelling south). So maybe it's better if biomes were distributed randomly.
I also hope that we will get some API for worldgen, in that case it will be possible to redesign biomes to more realistic or fantasy looking style, and add your own custom plants and objects into biomes. Personally I want to change caves and ocean biomes and add different fantasy plants to them 
It will be definitely possible to hook into the world gen process
There will be two ways to do that: One way would be to provide "raw" data, i.e. custom (2D) heightmap and texture data per "world part" (each world part consists of 512x512 blocks). The game would then use this data to create the world and generate the chunks.
This also enables you to control vegetation distribution. Unfortuantely we're still unsure about how to expose cave and dungeon generation this way... we also don't yet have an API to add or edit biomes (although you could create "fake biomes" by manipulating the texture data - for example, you could tell the game to spawn hell stone instead of grass etc)^^
The other way would be to provide the actual (3D) chunk data. This is a bit tricky to use though, but gives you full control over the world generation (you could ofc create custom caves this way) 