There are many "under-studied" philosophers at any given time or place. Let me give a few examples:
- In France, there are (mostly) not German philosophers that are studied, but their French interpreters (who at times vastly distort the message even when translating only), e.g. Kojève for Hegel.
- There is a revival of broad Kant-Scholarship of a new level in English-speaking academia since the Cambridge Editions contributed translations that are much more precise.
- There were times when the linguistic turn played a big role, others when epistemology was more naturalistic, including the corresponding authors being "in".
- There are often local research nodes, e.g. about Perennial Philosophy, Ancient Greek, Scholastic, Early Modern, Eastern, etc. that emerge because of a certain professor that educates and attracts specialists in the field. Often they are not even known on a broad scale, even if they publish.
That being said: I do not think there is any "under-studied philosophy" per se. This may be the case in certain circles/journals, certain historical periods, certain language communities. So the question can be broadly answered with "we probably do not even know 90% of the best philosophy ever written".
Also, academic philosophy is not about parroting or (re-)interpreting what is already written. That is one strand of philosophy, certainly, and mostly done in departments that work in the history of philosophy. The vast majority is about writing genuine philosophy using certain bits and parts of other philosophy as part of the argument, either to support or to dissociate one's own position.
Thus, what you learn in academic philosophy is not about parroting or re-interpreting other philosophers, but to express your own position by using arguments of other philosophers as either positive or negative foil (and the form of doing so without plagiarising). Only one aspect of this is knowing arguments of other philosophers in the first place. And yes, there are certain mainstream philosophers you learn. But everyone is free to choose to write about others (less/not known) as a postgraduate and many, especially more senior lecturers, do so.