After playing several times as the Ottomans, I have come to the conclusion that thier playstyle conflicts with their historical strengths. The conquest of Byzantium is straightforward enough, though the prerequisite conquest of Greece seems to be missing, and it is easier to conquer them heading straight for Constantinople with a few seige weapons. However, the expansions into Bulgaria, Hungary, Southern Kievan territories and Arabian Syria destabilizes my empire irrevocably, where these were actually the keys to the early and middle presence of the Ottoman. As such, the resitance to Ottoman expansion should be relatively weak early on, and strengthens as they move North and West.
Upon reflection, I offer these suggestions:
1-a collapsing state that submits itself to vassalage cannot lose its capital due to instability, player or not. At the very least, this could be a unique attribute for the Ottomans, who relied heavily on European subjects and Janissaries.
2-Hungary, Bulgaria and Kievan Rus become more prone to vassalize after losing cities, and switch to Islam upon this political change.
3-The Khazars, I think, should be implemented, as they were a very influential force in influencing the Byzantine Romans, and were responsible for stopping the spread of Islam into the Urals, and were the only European state in the middle ages to adopt Judaism as a state religion. This would bring the Kievan into a larger sphere of political influence, and not relegate them to some bushwacking fringe state that only exists to be destroyed by Mongols. After all, the Kievan is not able to successfully occupy foriegn lands, so the concept of Kievan Hegemony will lead eventually to collapse. Also, the Khazars themselves could become the Mongols if no state religion is adopted by 1100.

4-A Kingdom of Jerusalem should be established after a Catholic power occupies the city for X turns. The idea behind this is that the Crusades seem to bring about such instability that it is more useful to vote your enemies into positions of leadership, so they collapse soon afterwards. The frequency of the Crusades often leads to all of France, Germany and Burgundy as independants for large portions of the game.
5-The English should get +25vsMounted for Longbowman.
6-Mercenary nationality needs to change. I'm tired of seeing Ligurians in 1440. >_>
7-Venice and Genoa should have more initiative in destroying each other, and razing/capturing the other Italian cities.
8-France and Spain should have a stronger drive in capturing Italian cities. Naples for France, Milan for France.
9-The Islamic religious bonus should be scrapped. A common feature among Muslim nations was the Mamluk social caste. Perhaps a special Mamluk promotion (Start with March for unmounted, +1 movement for heavy mounted, maybe?) or a limited number of special Mamluk units available to Muslim nations, who go Barb when you switch? It seems the more I spread Islam currently, the harder it is to get my cities to plateau. There have even been a few Ottoman games where my largest cities grew too large, starved but continued to grow, got unhappy, revolted, and destroyed all my progress.
10-Castile and Leon should be seperate civilizations. Spain either grows too fast or too small currently, and I often have to resort to WB to ensure they dominate Iberia, because I get tired of shooing the Muslims out of Burgundy.

11-Catholic States that achieve a super-power status (Most often France, Germany, Spain and Austria) should have a new option to coerce weaker neighbor states into doing this or that, depending on the relationship. Burgundy to France, Poland to Germany, Portugal to Spain (Initially) and Hungary to Austria. Small things, like demanding unit renationalization (gifting) from vassals, cash or resource extortion through RFC choice menu instead of diplo, or giving up cities, regardless of whether the civ is a vassal or not. If the civ refuses, the Pope could issue a vote for war declarations, as the Papal state would be acting in a subservient manner, as a puppet diplo state.
12-The Netherlands should be able to spring up out of the instability of other civs in the region after 1150.
13-Resource placement needs to be changed. Why is there rice in Spain? As a tradeable, viable commodity, they should be placed as far east as possible, along with all silk, to establish the value of connecting to and maintaining a Silk Road.
Those SUGGESTIONS having been said, I would like to say that RFC europe has become my favorite mod, and I have stopped playing nearly all other mods/play styles. I am already flirting with the idea of using micromanaged saved CIV4 games to teach my classes when I'm teaching college in a couple years. Keep up the good work, and thanks guys!