The amount of newly available written premodern sources might be increasing because new translations, new discovered manuscripts, archeological finds, etc. Yet they might be decreasing because undiscovered and uncopied sources degrade and perish or just that we've translated a large part of written historical sources and are more and more scraping the bottom. In other words, is there: 1) an entire world of untranslated historic texts worked on by historians/translators and large number of undiscovered sources ready to be found? 2) some yet untranslated sources, but most have either perished, been translated, or are not worked on? 3) something in-between?
I apologize if the question is silly or badly worded. I'm not a historian and English is not my first language.
Well, judging from my field (Japanese history), it is, perhaps, even decreasing at below a snail’s pace when talking about records of various kinds, but increasing when talking about documents, since we still find more of those. The efforts of digitizing such old texts, and of producing edited volumes of all the handwritten stuff lying around in archives is proceeding, and I suppose that, eventually, we’ll have all of them available in printed and/or digital form. I doubt the situation is better in other fields: typically, historians who are being paid for simply preparing sources for the consumption of others (i.e., producing editions) are in employment in institutions located in the respective countries, and they do not see a need for translations to foreign languages such as English.
After all, professionals in the respective field can read texts in their original language anyway, and most historians who research, say, German history happen to be German (the same goes for Japanese historians working on Japan, US-historians on the US, etc.)—and most of them wouldn’t even bother publishing in English anyway (many actually wouldn’t even be able to, if they wanted).
Therefore, I’m really not sure why you even assume that translating historical documents is a common practice. Producing such a translation requires a lot of work if they are to be published: translation, annotation, and the writing of accompanying introductions describing the circumstances of their production and their significance. Someone has to pay for this. No one does.
But because professionals can read these texts anyway and because the overwhelming majority doesn’t even work in English, there usually needs to be an argument made to justify the translation, which is most easily the case with literary works that may have an aesthetic appeal beyond their functionality. (But even then, in my field, the manuscripts chosen for translations of those “literary” texts are the most popularized, and not the oldest variants, which would be more relevant to historians…)
As a result, most commonly translations of shorter works pop up as parts of a doctoral thesis, which would happen to be built around a central text that has not been extensively studied and translated yet in the previous (English-language) scholarship.
So, yes, there is an "entire world" to be found out there (the phrase you chose certainly encapsulates the scope of how much there is not available in English) .