There's a tree called a Sakura and I would like to add it to Rising World as a very tiny mod to help me get started on understanding on how to write mods or in this case we call them plug-ins
Basically this is possible, by "abusing" the custom 3d model feature for this (so as a result, the tree would not behave like other trees, and it's also necessary to place it manually [i.e. provide or calculate coordinates for every tree]), but there is at least no easy way like "editing the biomes definition so it spawns the new trees automatically"
Also creating a new biome is basically possible (you could go through the chunks and replace the terrain, i.e. replace sand in deserts by mud etc), but right now there is no way to "hook" into the existing pipeline.
However, I do not know if you can register a custom Inventory item (only in-game items are available without hacking) or modify crafting bench items
Right now this is not possible. It's planned in the long run, but I have no ETA for that unfortunately. It's our intention to have a way to replace existing items and recipes (by new models etc), but also to create own custom items.