Though I have always up to now considered French a very difficult Latin-based language, I have begun to think about taking up courses in French. It is a language of culture, melody, art, and it sounds musical and melodious too. I recommend it to you if you want to enrich your vocabulary and pronunciation. Note, though, French has a diverse linguistic source, so its vocabulary is prosperous. In pronunciation it is perhaps the most difficult language in the group of Latin-based languages, or those that have a Latin influence, as it is the case of English. English is originally a Germanic language, but if you give a piece of writing to a person who does not have a particular knowledge in English, he/she will distinguish it as a Latin language rather than a Germanic one.
Anyway, another language I'd recommend is Italian. Italian's vocabulary is not a stressful difficulty for you as a native English speaker, but if we are talking about its grammar, well... perhaps.... maybe... it is.
I've been studying English for eleven years, and this one is the twelfth. English feels to me the easiest language in the World because so it is. While about Italian, I've been studying it for about 5 years, but very sadly, Ive not been able to master it as I've done with English which I study in my faculty academically.
Cheers!