The links of our past are in Dacia Meditarranea. Just west of the Bulgarian capital there is a valley called Burel, seems to be related to the Albanian word berryl(elbow), the valley has a 90 degree angle turn, thus shaped like an elbow. At the heart of medieval Albania, in the Mat valley is the village/town of Burrel.
There many toponyms in this area of Bulgaria and on the Serbian side of the border that seem to stem from Albanian, and borrowed by the Slavs as shown by Slavic pronouncation. This is my finding by simply scanning the areas west of Sofia: Kambelevtsi (from Alb. kumbulla, plum), two villages named Dreatin (related to Albanian Drite(light), there are settlements in Albania named Drite),Bukorovtsi (from Albanian bukur), Arzan (same as Albanian villages named arrez, from the word arre (farm land)), Buzovitsa from Albanain buze(many such toponym derivates in Albania), Lyalintsi from Albanian lala(this toponym existed even in Peloponnese where Albanians settled near Patras), on the Serbian-Bulgarian border there is the Ruy mountain which can be related to the Albanian word for safety/protection ruj. Village of Giginitsi (gheg). Village of Oresha (similar to Orosh in Mirdite). Village of Bilintsi, could derive from blini. Village of Gintsi (probably from Gjin, which would equate to Ivan).
On the Serbian side of the border is the village Grapa (Gropa in Albania from the word for hole/depression in the land), village of Postalica from Albanian posht(lower), there are also two settlement with similar name Petrilje and Petrlas which reminds me of the Albanian fort and settlement of Petrela near Tirana where the first Albanian principality appears. Village of Dumbija(from the word tooth dhemb? like dhembel mountain), village of Lalinice again from lala just has a Serbian pronounciation. The village of Gaginice (gheg). Village of Resen (same as Reshen in Mirdita).
I am sure there can be more toponyms. But I more interested to look into any census data from the Middle Ages from these villages and nearby area. A proto-Albanian people clearly stayed behind and got assimilated over time but the Albanian heritage and names would have lingered for some time and would be easy to detect if we can get our hands such documents. I don't know if the Bulgarian empire(s) took any population census, if there isn't any, we should try to look into Byzantine records of the area and early Ottoman, because the Ottomans took over in the late 1300s and took defters right away.