Shannon, I've run two non-Imperial Traveller campaigns: the first was
before any information at all on the Imperium was ever published
anywhere (1977-1978), and the second was a "mini-universe" I whipped
up for a campaign for a course I taught in Role Playing Theory in
college (no, really! and for credit, too!). If one doesn't mind the
physics of the rules, one can rewrite things easily, and having a
different reality makes jumps like TDR a lot easier as well if one
DOES mind the physics of the Imperium.
There are a number of other folks who have their own universes too;
one fellow whose name eludes me at the moment has published a fair
amount on the subject.
metlay