I believe that when we worked on this project a couple of years ago, there are events are already in place for Trebizond to become the Byzantine Empire. However, when it comes to the Bulgarian Empire, by 1419, the amount of remaining Bulgarian nobility has been decreased, as the Ottomans had a practice of decapitating and/or eliminating rebellious nobles. However, in the example of Fruzhin, considering that in every historical rebellion, he needed Hungarian assistance, you would have to create events in Hungary seeking an opportunity for war on a weak Ottoman state, and then after taking the territories of Bulgaria, in-game, would either cede them to Fruzhin, or Sigismund would keep them for Hungary. A similar event chain could be put in place for 1444 when Wladysaw III attempts to Crusade on the Ottomans...However, just like the Crusade that never was, it never goes really far as Wladysaw III takes ill, and then dies in battle at Varna...you could put the events in, and create alternative endings, but the purpose is the restoration of one of the three Byzantine powers, to become the Byzantine Empire, either the Palagoi, the Komennoi, or the Princes of Moria (typically Palagoi who were uneligible for the throne.) If you can do it, go for it. Part of a mod is to create something that everyone would enjoy or play. EU2 variants that oppose the early Hegemony of the Ottomans are always interesting...