There are so many unwalked caminos in Spain, that there's plenty of room for everyone. If the Primitivo gets too crowded, there's the Vadiniense, the Invierno, the Olvidado, the Catala, Baztan, the Mozarabe... the list seems to be endless, fortunately -- or unfortunately for those of us who can never get enough!
But, Caplen, if you are starting to sink into a Camino addiction as I was back in 2006 when I walked the Norte, you might want to consider just sticking with the Norte the whole way into Santiago. Here's what I reasoned -- by that time, I knew I would be coming back to the Camino on a regular basis. I thought it was very unlikely that I would come back to walk Villaviciosa to Santiago (not sure why, it just didn't seem like a likely option to start the Norte "in the middle"), but that I would be likely to return to walk the Primitivo, especially since it can be walked in conjunction with the Salvador from Leon. The Norte beyond Aviles is really quite beautiful, too, and the walk through Galicia when you turn off the coast at Ribadeo is also very nice. Not trying to discourage you from your plans, because the Primitivo is one of my favorites, just throwing that out there into the mix.
And I hope you will let us know about the GR10 from SJPP to Irun, that is on my list as well, maybe in conjunction with the Baztan someday. buen camino, Laurie