Just out of curiosity, do you have much translation training? It's something you'd ideally go to school for in addition to learning the language. Of course this isn't always the case, but it's something you should consider, if translation is a big part of your dream. I recommend taking a look at /r/translationstudies to get an idea of what's involved. It's a discipline in its own right.

In addition there's more to consider than just what language. For example let's say you learn Classical Chinese, which was in use throughout East Asia for centuries. That doesn't mean you also know modern Mandarin, Japanese, Korean, Vietnamese or any other language of places that once used Classical Chinese. You'll need to know something of the modern language, because most of the texts you'll encounter will be held in collections in those countries, and you're going to need to be able to navigate the politics of getting access to those texts. I speak Mandarin and can read a fair amount of Classical Chinese, but that doesn't mean I can just walk in and start flipping pages of ancient texts.

Finally something to consider is your own interests. I can tell you that there are a lot of Tai manuscripts that are in need of attention, but that doesn't mean you personally are going to want to dedicate your career to sitting in the malaria-ridden jungles of Southeast Asia to work on those. Things like climate and culture should really be things you take into consideration.

All of these are things to keep in mind. There are manuscripts all over the world that need work, and very little has been translated into English. You almost have your pick of anything you can imagine.

The bigger issue is what you want to work on. It's your own career, after all.

Akkadian cuneiform is certainly a good option. There are thousands of texts untranslated into any language, let alone English. Akkadian cuneiform, in addition to Akkadian/Babylonian/Assyrian, gives you a basis to write other languages written in the same writing system, such as Hittite, Ugaritic, or Urartian. You can see some of these online as well at CDLI.

John Huehnergard, A grammar of Akkadian is a very comprehensive grammar if you're interested. It will teach you both the writing system and the language, though it might seem like a bit much, and isn't particularly user friendly.

I'd also recommend Snell's A workbook of Cuneiform Signs or something similar to practice the signs. You could alternatively use this in combination with a simpler, more user-friendly grammar, say Worthington's Complete Babylonian, which is actually incomplete because it doesn't teach the signs!

Good luck!