Adding an NFA test to demonstrate checking for parenthesis balance - shows the limitation of NFA (and regex) for these types of situations.  We have a finite number of rules, so we can never build enough rules to safely check for balance (can always have one more set of parens than we do rules).
Adding basic support for "Deterministic Push Down Automatons" (DPDA).  This involves creating a new "Stack" concept, a "PDA Configuration" (the mash up of current state + a stack), and a "PDA Rule" (which is almost identical to an FA Rule, except that it has the extra feature of "pop" and "push" characters for altering the stack upon state change).