DSLs and Scala? That’s a huge topic in EPFL. Loads of theses on it. You might want to start from that of Gilles Dubochet [1] or Eugene BURMAKO [2], for example. And, if you’d also be interested in an unorthodox viewpoint, you might want to try my thesis [3].
From what I understand, this topic is so broad that if you don’t narrow down the domain, even an extensive survey could constitute a good Master’s thesis.
Good luck,
–Hossein
[1] https://infoscience.epfl.ch/record/163536/files/EPFL_TH5007.pdf
[2] https://infoscience.epfl.ch/record/226166/files/EPFL_TH7159.pdf
[3] https://tubdok.tub.tuhh.de/bitstream/11420/1220/1/Thesis.pdf