I seriously doubt the Ottomans would hit a brick wall. The Serbs didn't expand because they were super strong but because the Byzantines were very very weak.
The Ottomans were no where near as strong as they would be until they had cemented a power base in the Balkans, which took them a solid decade to do just in Thrace. Prior to Murad's ascension they were another beylik in the region that was weaker than their surrounding neighbors.
Any intervention before that region was conquered and colonized by the Turks would not be difficult. Their troops at the time were largely unprofessional, consisting of Ghazi troops whose morale and loyalties were questionable the minute conquest stops flowing in, and the Janissaries consist of about 1,000 trained boys who lack their famous gunpowder weaponry.
Now while I won't overestimate the Serbs, its worth noting that under Dusan they routinely beat the Hungarians, the Latin troops in Greece and any remnant Byzantine forces.
A better question to ask would be, what would the world look like if there was no Serbian empire. Would the Byzantines be more stronger to withstand the Turks.
Depends on who wins the Palaiogian Civil Wars, of if they are prevented. The Turks don't have a navy, and the only Anatolian power with a fleet worth anything was allied to the Byzantines at this time, so if they aren't invited in they won't be getting into Europe at all.
Dushan was a flash in the pan. Serbia could not hold out long in all likelihood while remaining Serbian, and the new regime would make more enemies the further it spread. .
His untimely death made it that way, but the state had potential to last. During his later years he began to essentially Byzantinize his state, copying their rituals and administrative practices. Most likely, the empire doesn't stay Serbian as we know it, but takes greater and greater Greek influence as the administrative apparatus is packed with skilled bureaucrats from Byzantium.
The regime was already unliked by all of its neighbors sans Bulgaria (who is currently rapidly decentralizing and thus being neutered as a threat should they decide to turn against the Serbs.)
That is also the problem. Nonslavs, barring Albanians, will be very tough to assimilate and serbianize
edit:
the only direction Serbia can expand is Bulgaria (linguistic differences back then were minimal) and after that into Moldova and towards Crimea, but that would piss off Hungary who had interests in the area
Wait, why do linguistics matter in a pre nationalistic era? Religion is far more important than ethnicity in this time period, and all the region sans the Croatians share Orthodoxy (and that can change).