I also considered the 'Blub paradox' that Paul Graham talks about. I asked where the different programming languages fit on the "power continuum" - that is, its features, what's present and what's "missing", and how I should weigh each when thinking about that next language to learn.

Along with power, I like the view on utility.

As you write, thinking about your immediate problem and future problems (i.e., if you could anticipate) to solve, are also good points to consider when deciding about that next language to study.