I really look forward to the day when it's easy (and safe) to have API's at places like Vanguard so developers skilled in areas such as UX can layer technology solutions on top of the fantastic investment products provided by outfits like Vanguard.
On a slightly related note, I am working on the trading part mentioned in the article - i.e. APIs for advanced methods of getting the trades done. Basically, these trading methods are use machine learning to predict near future movement and use that to optimize the orders. (Some more details here - https://www.qleap.co/ - in case, someone is interested.)