It should be noted that the Birthright Boxed Set gives some rules for
creating your own world. They are not under the "creating your own
world" heading, but there are some qualifications for determining what
holdings can attain what levels and why. i.e. - only exceptional
holdings can have a total domain and source level higher than 7.
Using some of those guidelines from the Boxed Set, I would think that
a Birthright-type campaign can be run in just about any type of world.
There are some exceptions, or some worlds that would be to difficult,
but if you have the sets mentioned in previous letters (Greyhawk,
Al-quadim, etc) you should be ok.