I'm a proud Latin student and not only can I say it's a beautiful, fascinating language, but that studying it has truly given me a strong base when learning many other languages. After studying Latin I found myself capable of roughly understanding foreign dialogue in film and literature using what I'd learned from that one language and I think it's one of THE languages to learn. I'm surprised it's not on Duolingo already. I mean, we have Klingon! Not hating on Klingon, Klingon's awesome, but if we have that we've gotta have Latin. Who's with me?
Kial estas ĉi-tio en la Esperanta forumo? Vi devus movi ĉi-tion al la ĝenerala forumo.
If you put Latin in the forums' search box you will find that it's been suggested several times before. There is also an option to apply for contributing to a Latin course if your skills are good enough, but there's no way of knowing when a course will be made.
In the meantime there's always Romanian and the other Romance languages. : )
Duo does not have Klingon. True it is in the incubator but it has not been released yet and may not be released for several months or even more.