The Latin Kingdom of Jerusalem changed dynasties more times between it's establishment and fall (1099 and 1291) than most European states did in the same time. In part, this was due to the near-constant female successions: in those 200 years, Jerusalem had five queens-regnant (Melissende, Sibylle, Isabella I, Maria and Yolande/Isabella II), each accompanying a dynastic change.
Not only that, but there seems to have been an appalling lack of boys - since most of the dynastic changes didn't last much longer than a generation (Isabella II was the only child of Queen Maria, who in turn was the eldest daughter of Queen Isabella I). Melissende had one son, Sibylle had one son (who died young); Isabella I only had a son by her Cypriot husband... So, as a result, Jerusalem saw a succession of well-connected minor nobles marrying up to royal rank.
But what if the dynasty was more secure? Let's start it from Melissende (since she was one of four daughters), and her Angevin marriage. They had two sons (one who died childless) and after that the succession got progressively weaker. Baldwin IV had leprosy and never had kids, Baldwin V died young. And there was disrespect of Baldwin IV's wishes as well - since he had directed that Baldwin V was to succeed him, and should the new king die young, the European powers were to decide whether Sibylle (Baldwin V's mother and Baldwin IV's sister) or Isabella should succeed.
So, let's go with Baldwin V surviving. Him surviving immediately weakens Isabella (and her kids from her various marriages)'s claims to the Jerusalem throne. However, they still have a (strong) claim as long as he's without heirs. So, I'd imagine he'd need to marry pretty quickly. However the difference between him marrying and one of the queens marrying - with a female ruler there was always a squabble about who to marry her to (with no noble family wanting aniother to be more important than themselves). The marriage of a king (while plagued to by the same pitfalls - should he wed locally) can be used to build alliances (whether with European states - in a way that a queen's wedding could not; or with other states in the area - Byzantium, Armenia, Cyprus, the Latin Empire etc).
Thus, my WI is based on the idea of Baldwin V surviving, and thus denying the foreign powers the chance to meddle in Jerusalemite affairs as they had a tendency to do OTL. Can he marry a European/Byzantine princess? What kind of good would it do? Especially if the kingdom of Jerusalem were to pass from father to son until 1291. (After that it's open season).