That teritory is just part of modern Macedonia:
Prince Marko was not able to fight for Serbian throne, since Mrnajvčevići army was anihilated in battle of Marica 1371. Lineage of Mrnjavčevići is from Zahumlje, he was probably born arround Trebinje (first mention of King Vukašin denotes him as župan of Trebinje is from 1346, Marko was born 1335). Marko was regarded as a man of trust by his father's co-ruler. He participated in state affairs (Negotations with Dubrovnik). He lived in modern Macedonia, but he considered himself legitimate Serbian ruler, I repeat, all of Macedonia, Northern Greece and large partions of Albania were under rule of Serbian feudal lords. As born in other part of Serbian state, being etc, he could not be considered native (ie Macedonian Serb). I am aware that there are poems about him in Macedonina, but honetly, it could not be even compared with number of Serbian poems about him.
I am sorry, but must object that statment. You could see text here: http://www.dusanov-zakonik.com/ There is no reference to any other nations except Serbs and Greeks in Code.
His title in Greek was: βαδιλευς και αυτοκρατωρ Σερβιας και Ρωμανιας, in Old Church Slavonic his tile was longer:Азь в Хрїста Бога благовѣрни и Богомь поставлени Стефань, царь вьсемь Срьблѥмь и Грькѡмь и западнимь странамь, сирѣчь Арбанасѡмь, и Поморию и вьсемь Дисоу.
There was no any mention on Macedonia in his diplomas, nor in code.
Well, I repeat, I am not against Macedonians and Bulgarians seeing Marko Kraljević as part of their legacy, but he was not Macedonian nor Bulgarian in any sense. Politicaly (Serbian prince and statesman), Sociologicaly (member of Serbian nobility), Religously (he acknowledged Jurisdiction of Peć Patriarchate) ethnicaly, he was Serb and furthermore Serbian Serb.