Various Turkish emirates unite and defeat the besieging armies.
That's not going to happen, at the very least, not that soon.
Turkish emirates and sultanates (and neither other Muslims states in the region) really grasped the Crusader motivation, or even distinguished them from the various Latin mercenaries of Byzantines.
They simply didn't see that the whole expedition was going its way East and South, possibly threatening them, and even when they did see the danger (as in Antioch), it didn't prevented them to maintain a political and strategical disunity.
Asking them to unite, after the first expedition was crushed easily and while Kiliç Arslan is in an ongoing war in Eastern Anatolia, would be really pushing it.
The siege of Nicaea which was undertaken by Byzantines and Crusaders fails.
It would be really hard. There, you have the maximum number of both Latin Crusaders and Byzantine reinforcement you'd ever had in the First Crusade.
The only thing I could imagine would work would be an epidemic in the Christian camp, but even that would take simply too much time (the city was taken in one month) and could weaken besieged as well.
Another way would be to butterfly away entry of Latins in Anatolia, making troubles among them going to a whole new brand level. It wouldn't prevent Crusaders to advances (Raimond of Saint-Gilles would probably pull the same trick than when he left Antioch), but could reduce numbers.
That plus epidemics could *maybe* do it if you remove Byzantine assisstance as well.
For the latter, it's being complicated. Latin forces were something Alexios needed bad, to take back at least token parts of Anatolia in order to not piss too much anatolian elites that already frowned up on his abandon of Asia at the benefit of Balkans.
Him being replaced in the late 1090's could be a good PoD.
The consequences would be interesting.
- Byzantine Empire would sooner or later take back coastal Anatolia, would it be only to inner politics matters
- Rum Sultanate could expand its influence eastwards more importantly than IOTL, including part of northern Iraq. The butterflies would be huge for the region, of course, so it's hard to get a real idea of what would happen long term
- Fatimids could have a better decline, comparativly to IOTL, less plagued by assassination and revolts.
- Changes in Latin Christiendom would be limited, at best. The changes would be essentially cultural, and institutional (no use of Crusade as a political motivation); and *maybe* a stronger focus on Spain, Italy and Africa?