I find this topic very interesting. (:
In my opinion, if linecrossing is done then the genepool is not too small (though since I'm no breeder this is just a consumer opinion- I have met quite a few mixes of WGSL and DDR/Czech lines lately whom I have found quite remarkable).
However, if outcrossing were necessary to save our beloved breed or simply to create a new breed that is more stable... I do have a few breeds I think might work. Of course, the dogs of these pedigrees would all still need to be health, temperament and ability tested just like any purebred shepherd breeding.
To stabilize the temperaments a little more, into a less reactive dog, I believe I would add some Maremma sheepdog into the lines. Because they are a lesser known breed they have had a little less exposure to crooked breeders, at least in the US (to my research thus far), than other breeds. They are also not much larger in standard than shepherds. However, this crossing could bring about a more dominant/independent dog. I have found no commonly occurring health issues yet, but hip displasia would be a concern (as well as a few others that I just haven't unearthed yet).
Aside from the Maremma, I may breed in some American Indian Dog. I chose this breed because, like our beloved shepherds, they are highly versatile. They are reserved of strangers but are not meant to be vicious. Introducing their bloodline would, however, truly require an experienced and in control handler as well as very careful and consistent socialization. The breed has no breed specific genetic issues, and is not easily found so hasn't really been bred into smithereans (sp?). They are a little smaller than shepherds, but not small enough that I would expect "puny" offspring to result.
My last breed that comes to mind is the Karst Shepherd. Again, they require knowledgeable owners. However, if raised, trained and exercised correctly they are known to have very stable temperaments. Like the other breeds I have chosen, they have little exposure in the US and are generally tricky enough to find that there has been minimal damage to their lines. They are usually between 58-88 lbs so their size ranges in the shepherd's standard. I have not found any evidence of breed-specific genetic issues for these guys, either.
So, IMO, shepherds do not need outcrossing with other breeds. Other lines maybe, but not other breeds. And honestly, if all good breeders expand their range for breeding (even in their lines), we may have no issues at all with the genepool. But, those are my three top choices for breeds that I would likely agree to outcross shepherds to. OF COURSE: This would not just depend on breed, but also on the specific dogs being brought into the breeding and their pedigrees. (:
Mom To~ Dakoda (b/t GSD), Acheron (black GSD)