I think the most important thing is that your portfolio app actually do something useful to (at least) you. If you invent some problem to solve you’re also going to invent features which won’t take you anywhere interesting. With even just one customer – even if the customer is you – this changes. It also ensures that you’ll want to work on and complete it. Find a problem and solve it. Have 1000 people already solved it? So what - you can do it the way you want it done.
When I review someone’s sample code, I’m not looking to check off a list of rails features that they’ve used. Mostly I’m looking to get a sense of their style and some areas of the code I want to ask follow-ups on.