I think the Intellisense feature should work for :WaitForChild, :FindFirstChild(), and variable names.
For :WaitForChild() and :FindFirstChild(), it should only work if Studio can immediately find the objects.
Sometimes with :WaitForChild() and :FindFirstChild(), I replace them with the dot notation quickly to view the properties, then retype them. Adding intellisense for these would solve that issue.
For variable names, I mean when you type the variable name with a dot, if Studio knows what the variable is defined as, it should show the intellisense for the properties of that object as well. Sometimes I type the full hierarchy of the object just to get to the intellisense, similar to the :WaitForChild() and :FindFirstChild() issue, and that would be solved too.