Hmm.... some interesting ideas. A few thoughts:

1. I wonder how the "politics" of the era could be enacted in this mod. In other words, if you're thinking about factions like, say, the crusader kingdom of Jerusalem, as well as France, what happens when the king of france wants to go on crusade? MTW diplomacy being MTW diplomacy, I imagine there could be a lot of situations where the French end up allying with the Saracens to beat up on the Crusaders -- which is fine I guess, but it might not be the kind of game some people want.

2. What kind of new map are we talking about? First let me say that I have NO experience making new campaign maps, so I would be no help there. But what did you have in mind for the area of the map? Personally, I think it would be great to have Constantinople east to the Caucasus (basically it could end where the MTW map ends on the eastern side). That way you'd have Constantinople itself as the seat of the Byzantine Empire, and you'd have at least a chopped-up Byzantine Empire, its land holdings depending on the start date, which would make things interesting. Also it sounds like you want to include some of the eastern Mediterranean islands, which sounds good.

4. Any ideas on factions/units?

3. Is this mod really feasible? I don't want to sound like a pessimist but the more I think about it the more interesting this seems, and I'm just wondering how willing people would be to work with this. I could offer some research time to help out myself, and I know a bit about basic modding (I've done some campaigns for myself in the past), but you'd need someone who can work with the campaign map and maybe make brand new unit graphics and shield graphics -- all of that stuff is beyond me.

Well it sounds interesting -- keep us posted.
CountMRVHS