1) Is it possible to make go to symbol within project pop up previews?
2) Is go to definition supposed to learn from previous user selections?
3) Would it be possible to hook into go to definition to filter out irrelevant suggestions?
4) How do I ignore certain folders in the sidebar from being indexed?
The GotoDefinition command is defined in a file symbol.py in the Default.sublime-package (which is a zip file) in your installation folder (C:\Program Files\Sublime Text 3\Packages for Windows).
So I suppose you can copy and paste the content of the file in a new file of your User folder, rename the class and adapt it to your need.