What auxiliary language or constructed language (conlang) would you like to learn and why?

I can’t count Esperanto, since I have already been fluent in it. Nor Klingon, ditto. Nor Lojban, ditto.

So let me go through the others, and say why or why not I’d like to learn it, if I was 20 again, back when I had the free time. Ranking from less to more.

• Láadan. Pfft. Hectoring mental straitjackets: not my thing.
• Toki Pona. Meh. Cutesy mental straitjackets: not my thing.
• Basic English: Nah. Disingenuous in its execution: too much English idiom in its phrasal verbs to count as truly minimal. I have a bit more time for its modern descendent, xkcd: Up Goer Five.
• Ido: No. Very close to Esperanto, and where it’s different, I didn’t like it: it was neither fish nor fowl in the schematic/naturalistic debate.
• Loglan: No. One logical language is more than enough.
• Novial: No. Rather more naturalistic than Ido, but never attracted me. Probably too much Germanic.
• Tolkien languages: … Nah. Lots of philological cuteness, but ultimately not enough vocabulary there, and too many gaps to be useable.
• Talossan: Almost yes. In fact, I was approached by King Ben way back to join the community. Given the ensuing shitfight in the micronation, I’m glad I didn’t.
• Dothraki: Almost yes. I tried in fact, but the vocabulary just wasn’t there, either. And the fact it got killed off in Season 2 enraged me against continuing it. (I want NO SPOILERS about Season 6. I watch Game of Thrones on DVD.)
• Volapük: Weak yes, for the cuteness factor of all those moods, especially in its baroque original form, as opposed to the stripped down post-1931 version.
• Occidental: Yes. It was the best of the naturalistic languages, as the closest approximation to the pseudo–Franco-Italian they were ultimately going for. I enjoyed reading through the back issues of Cosmoglotta—although it got nasty towards the end, when they were gloating that Esperanto had been banned by the Nazis, and then got banned themselves.
• Interlingua: Yes. Like Joachim Pense said, all the good bits of Latin. Probably more Peano’s Latino Sine Flexione than Gode’s more Vulgar Latinate version which won out; but I’m delighted on the rare occasion that I use software with an Interlingua interface (Mantis Bug Tracker).
• Interglosa: Yes yes yes. It’s less neat than I remember it from when I first came across it; but its attempt to reduce all verbs to a dozen or so verb valencies plus adverbs make it a thing of beauty. And I rejoice that Xavi Abadia has unearthed the unpublished Interglosa dictionary, and put it online.
• Not to be confused with its epigone Glosa, which takes out much of the good stuff.