Nearly all ideas are possible to create as a mod as mods have access to all the games code it just depends how much time the mod creator is willing to invest in learning how. Some things such as creating custom vehicles require a lot of time to understand how the developer created things and using that to implement your own things.
To create a mod you just need to know how the Unity Game Engine works in C#.
Everything is coded in C# just without the editor, you can also import assets using asset bundles which require the editor to create. More info can be found on the creating a mod page.
I have no clue when it will be available, it is a large task. It requires a lot of reading and understanding how the game works and then writing the code to get it to work. Multiplayer is more of a fun project, I'm not expecting to get fully working multiplayer system in place. Multiplayer officially by the developer is possibly getting implemented in the later future after release of the game anyways.