Man, I'm having a blast as King Uros the Strong in a 1337 (lastest start) playthrough. Snagged Epeiros and Styrmon off of Epirus and Bulgaria, respectively, then helped the Byzantines against an Ottoman holy war for Thrace, then swore fealty to Byzantium because of roleplaying.
A Bogomilist rebellion destroyed the Bulgarian army, and the Duchy of Moesia was suddenly ruled by a new Bulgarian Bogomil count. I immediately declared a holy war against him, usurped the Kingdom of Bulgaria, and took Vidin to make my borders look nice.
By this this time me and my brother are both married to Byzantine princesses, and my son is betrothed to one (his mother-in-law and aunt's sister). Then all of a sudden I become ambitious and zealous and--why my oh my, the Byzantine emperor has become Catholic somehow.
So I declare a war to depose him and put his son, my daughter's husband, on the throne. I win handily, crushing another Ottoman holy war just outside the walls of Constantinople, take three holdings in that county, and force the Catholic to peace.
Then I fabricate a claim on the Byzantine Empire through plots because, fuck it, why not? I directly own more than half the country and control two thirds. And my son-in-law folds to my demands and suddenly I find myself Count of Constantinople and Emperor of the Serbs, Greeks, and Romans. Fuck yeah.
An actually competent holy war for Nikea against the Ottomans followed. Things are looking good. Except for Venetian Ionia and Frankokratia Greece, but that can be dealt with.
Funnily enough Venice lost the City of Venice early on to a mercenary invasion, the Aztecs (yeah, whatever, it's fun and mixes things up a bit) are doing more harm to the Muslims in north Africa than anybody else, and the HRE has been in a state of constant civil war for the past 20 years. Oh, and the King of Sweden is also the King of Norway, a realm completely united.