Deciding on which language to learn?

I know a few languages and I think I'm ready to begin tackling another. I'm not hung up on practicality, but rather a language which I have fun learning/using. My problem is that I believe I should pick whichever I'm impassioned to learn, but that "passion" can change on a daily basis. To date, all the languages I've learned I got dedicated to because I managed to get a good amount of work in before I could change my mind, and liked what I'd seen so far so decided to stick with it. On a regular basis, there is a list of languages that pop into my head–Esperanto, German, Dutch, Catalan, Portuguese, Romanian, Greek, etc. My problem is deciding /which/ I want to try next.

If it makes any difference, I know English, Spanish, French, and Italian (at varying levels of fluency) right now. Any advice on how to pick? Any experience with those languages?

Note: Like I said, practicality is nearly irrelevant to me; I'm doing this for fun, not for usefulness.