It's a Vague title I know, but hear me out.
When Prince Carl of Denmark became elected King of Norway in 1905, he decided to rename himself Haakon, as it was a historical name borne by many Norwegian kings. But, doing this also gave him the Regnal Number VII. So my mind started thinking "What if the newly emerged Kingdoms/Principalities in the Balkans did the same for the same reasons"
1: Bulgaria.
First off, Two of Bulgaria's monarchs took their name and regnal number from past kings. Boris III and Simeon II (and the Heir apparent is named Kardam), but I want this to specifically apply to Ferdinand I as well. So, I decided that ITTL he renames himself to become
Peter V.
2: Greece
Now George I of Greece actually did rename himself from his Danish name Vilhelm upon becoming king, so with that in mind, it shouldn't take many butterflies for him to rename himself after one of the Byzantine Emperors. My pick was for him to rename himself John and take his regnal number from the Byzantine emperors to become
John IX. Without a George I, there would be no George II so he would be
John X.
Alexander of Greece can actually keep his name, he would just get the number II behind him, as there was one, rather obscure Byzantine Emperor by that
name.
Now what you probably waited for the most, the two Constantine's. Yes, ITTL they do take their regnal number straight from the Byzantine emperors to become Constantine XII & XIII respectively.
3: Serbia
Peter I of Serbia would simply get the number II behind him, as there was one Serbian monarch by that
name prior to him. The Alexanders, however, are a different story. Here I did what I did for the other monarchs and found out that so many of Serbia's Monarchs were named Stefan, without anyone of them numbered (Except for 5 whose full name was
Stefan Uroš) So, I decided to cheat a little bit by counting every single Stefan up (excluding the aforementioned 5) and included all the Alexanders to end up with Stefan XII, XIII, & XIV respectively.
4: Albania
The Monarch William also gets an entire name change, but since he's of German origin, and his house was called Wied-Neuwied. I think it would be perfect to simply give him the name, Carol (Charles), as that, would give him the regnal number IV. since 3 Albanian monarchs prior had that name. (Ok, they were all not actually Albanian, but they're counted as "Albanian Monarchs" so...)
So yeah... How likely would all of this have been?