This is the premise, you are going to write a new XWiki.
You are not bound by any backward compatibility requirements.
Use any technology or combination of technologies.
PHP? C++? Golang? Node.js? Java? Your call!
Describe it in as much detail as possible.
What frameworks will you use? What ORM? what databases?
Why will it perform well?
How will you get new features into the hands of users quickly?
How will you avoid this:
http://steve-yegge.blogspot.ca/2007/12/codes-worst-enemy.html
and this:
http://www.laputan.org/mud/
Think big and go wild!
Thanks,
Caleb