If someone wants to dive into the JS parser, they can see Gabriel Wicke's notes and TODO list here:
https://www.mediawiki.org/wiki/Future/Parser_development
There are also lots of interesting links on Gabriel's user page:
https://www.mediawiki.org/wiki/User:GWicke
In case you want to see the VE team's TODO list:
https://www.mediawiki.org/wiki/Visual_editor/Todo_list
and their notes on currently deferred, broken, or incomplete features:
https://www.mediawiki.org/wiki/Visual_editor/Phase_2