I would like to see Git be extended with a decentralized approach to bugtracking, code reviews, wiki's for documentation etc.

Isn't that Fossil?

https://fossil-scm.org/home SCM, bug tracker, wiki...

What do you need for wikis? Just put text files in a folder?

Bug tracking in git: https://github.com/MichaelMure/git-bug

Code review in git: https://github.com/google/git-appraise