Great work. Thank you for sharing it. You'd be surprised how hard it is for big companies to make a tool like this.
Ideas to play with: 1) Have one of the outputs be a box and whisker output showing the percentiles of ages the person will run out of money.
2) Have a way a person could easily code a transition from an S&P500 portfolio to a fixed income portfolio over 5 to 10 years.
3) Add a generational mortality table on the back end and include a probability of death.
edited to clarify (2)