d20 Classes for SF

This is something that has been giving me headaches ever since starting my d20 conversion of SF.  The d20 system pretty much requires character classes to oeperate; in classic SF:AD there are no classes, and in Zeb's Guide there are the four professions (enforcer, explorer, techex and scispec).  What I have been trying to decide is whether to use the d20 Modern classes, the Star Wars d20 classes, the T20 (Traveller d20) classes, or write my own based on the Zeb's Guide.  I find the d20 Modern classes are just too basic, the Star Wars classes too focused and the T20 classes to boring, but it would make things easier to use one of these systems rather write something new.  The short-lived Alternity game is close to the d20 system, and the classes there are very similar to the Zeb's Guide, so that is a template for possible conversion.  Has anyone else worked on this, or have any opinions?

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