the funniest part about the "TRUE ROMAN SVCCESSOR" state thing is that the country that probably has the best "legal" claim to the title also 100% does not claim it, don't mention it, don't even pretend it was ever a factor.
It's Spain.
Andreas Palaiologos bequeathed the titled to Felipe and Isabella of Castille. Although that remains contingent on Charles VIII of France's purchase of the title being invalid (I would argue so, seeing as he failed to live up to the stipulation of the sale)
The Turks invited the Ecumenical Patriarch of Constantinople over to make it official, so if you're one of those "The pope appoints the Emperor" guys then sure.
I've seen unironic arguments that the Roman Empire, HRE, British Empire, etc should all be basically considered dynasties of the same empire the way the various states that ruled over China all get treated as dynasties of one empire, though I believe at least some of that particular point is to highlight how eurocentric historiography is all too willing to endlessly subdivide and see distinctions in European states while elsewhere it just sees one mass of people that's just ruled by one family or another. Though I believe the modern historiographic standard is to apply the same level of nuance and detail that historical European states got to other places.
how would it be serbia? the "real" succession goes rome -> constantinople -> moscow. then due to the bolsheviks, the only part of the former russian empire to escape the red peril and remain a viable successor is fascist finland.
Tone clarifyer: Its a joke. I'm referencing an old meme/injoke that came about after people noticed some people from Serbia would claim everything and everyone as being Serbian - Elvis was serbian and "Tupac alive in Serbia" are probably the most famous of these.
It's sort of related to "greater Albania" memes that also rely on a weird niche group making strange ahistorical claims. See also: "come to Brazil"
also, this thing about claiming everyone is not unique to serbia, it's extremely widespread from slovakia on down the balkans.
i once stumbled into the site of an earlier wikipedia edit war on franz liszt's ethnicity. hooooooo boy. everyone from the former austrohungrian monarchy was there, trying to claim him.