1)  There are quite a few models that we would gain by having both.
2)  We would benefit from the bug fixes and scripting commands gained by each expansion.  Things like pictures in books don't seem to work unless you have all the expansions.  Very powerful scripting commands are included in the expansions - things like weather control and too many to list.
3)  We could not decide which expansion would be more beneficial if we only did one and we really want what both have to offer.

Since this mod will not be released for a year or more, and most people should have the GOTY version or the expansions, we think, we want to take advantage of all the expansions.  

Also, since all your development will be done in CS without the Morrowind, Tribunal, or Bloodmoon esm's checked, it will not be impossible for those who don't have all to work on this.  There will be a problem of not all the items will show properly and errors will show.  And no one who doesn't have all will be able to write scripts.  Not sure what else is a problem.  But we are basically requiring those who work on the mod to have all expansions.

Opinions?

This decision needs to be determined prior to the first city being done.  What many cities look like.  Many cities will be different as time goes by.  New buildings, burnt out buildings, etc.

The following items are determined by how much time our mod will encompass:

1. Rumors that the player hears.
2. NCP location, friendliness, status (dead or alive) etc.  
3. Ruling body and military occupation for nations and cities.