The other day, I was wondering about this, and I hope I can put this down and it's understood.

If I were to breed (as an example) an Arab-lined Arab to a Morgan to get a Morab, then breed the Morab to an Arab...would I get a crossbreed or a Morab?

Like...is it possible to breed back to an Arab line again? Kinda like RL, where outside breeds have been added to another to add traits or improvements, but the descendants are not considered crossbreeds, but of the original breed? 

As it currently stands, morab x not a morab would be a crossbreed (according to the crossbreed list in the help pages).