While I would love to work with some programming languages full-time I know that it will unlikely be the case. However, being able to split my job into smaller jobs seems like a more feasible idea. My bread and butter is Dynamics NAV, but I also like working with Ruby, Go, Dart and Coffeescript.
This is where having a portfolio career would be nice.
Working a couple of days a week doing Dynamics NAV work, then a couple of days a week working with Ruby and then hopefully leaving myself the Friday to work on my own products would be a great way to balance the week out. It also means of course that I keep a broader range of skills updated rather than focusing on just the one.