First thing, be patient!
It takes time to learn a new framework, changes, and developing plugin, all are in hurry to convert their plugins to newer version, but really think to give your hand to make newer version more stable first!
Let's come to the questions:
MagicMax wrote:what type of project should i use in order to create a new plugin? I understand that it should be class libriary
It should be Class Library(.NET core)
MagicMax wrote:and the documentation (for developers) is out of the date!
Not at all, it's updated as per the last release 3.90
MagicMax wrote:also i can't develop any new plugin for the new version of nop commerce...
You can refer any default plugin to develop your plugin, and it's pretty much easy, even if you are not able to develop..leave it for a now, As Mr.Sohel said
Nop Team will release a documentation soon for nop 4.00MagicMax wrote:i'd say that IMHO it's not good idea to jump to .net core as it changes a lot of things in the solution
May be, but .NET Core is now more matured!