"I would contend that feudalisation as represented in CK was only present in a few areas in 1066 ... Most of Central and Eastern Europe was tribal and the structure of the Roman (Byzantine) Empire was significantly different.
Hence I would consider the appropriateness of the CK model to be diminishing rapidly at earlier dates."
First of all how do you define "feudalisation as represented in CK"? Basically I see it as the system of vassalage with counts/equivalent holding power under dukes/equivalent holding power under kings/equivialent. While this could certainly use some tweaking, this situation is demonstrably present in several areas of western Europe by c. 700. Granted, Eastern and Northern Europe were quite different- but similar systems of "tribal" rule existed in parts of the CK map until after 1453. Likewise, while the Byzantine and Caliphate governments did have significant differences to the governments of western europe prior to 1066, these were still in place in one form or another until 1453. Moreover, city republics such as Venice had a ruling system somewhat alien to "feudalism" and are present throughout the timeframe of CK. What I'm saying is that these non "feudal" governments are not unique to the period prior to 1066 and while the research for earlier start dates would be tricky, the engine could represent them reasonably well.
"I think starting with the 1st Crusade has a lot of merit, because this crusade differed in kind from the later ones...The current crusade model is a better match for the later crusades."
I agree that the crusade model needs an overhaul, especially if it is to remain the focus of the game. (Although I do have some issues with your analysis of why the 1st crusade was so different from later ones). I see two problems with starting in 1071 though:
1) the Byzantines will be very weak at this point and will either need some advantage (great ruler could do it, or maybe divisions amongst the Turks?) or will be steamrolled by the Seljuks in most games.
2) this start date is after the Saxon rebellion has been put down. I don't think you can underestimate the appeal of playing the period 1065-1071 for a lot of Brits/Americans. 1066 is often the only date people know about the period. Yes, this is an Anglo-centric consideration, but it's a valid one nontheless.
Maybe have 1071 as a date availiable to play from, but I think you still need to keep 1066 in there.