If they went the route of an Old Republic era product, I bet it would be a full-fledged core rulebook. There would be so much material to cover, they'd almost have to make it a huge book, and since they've shown a willingness to do a bunch of core rulebooks in the 40K line, I would not be even slightly surprised if we eventually ended up with 4-6 core rulebooks in the Star Wars line as well, accounting for possible second editions such as Dark Heresy is getting.
I would absolutely buy that.
The difference between the SW and 40k lines though is that all the SW games share the same system and are meant to be able to be played together, while the 40k line is not.
Well, the 40K books use the same basic rule structure, but each book is for progressively more-powerful character tiers, from the scrubs of Dark Heresy to the rather burly Space Marines of Death Watch, and the core issue is that the system really wasn't designed to handle such powerful characters, instead being an adaptation of Black Publishing/Green Ronin's WFRP2e system that FFG was required to stick with when they took over the Dark Heresy product line.