Leave the Camino Francés at León and walk along the Camino del Salvador to Oviedo and then take the Camino Primitivo via Lugo to Palas de Rei (connection to the Camino Francés). In the Middle Ages this was a classic "detour", under the slogan: "First see the Redeemer (= San Salvador, the Cathedral of Oviedo), then Santiago". In these days the Camino del Salvador is not used very often, the Primitivo more and more.
Leave the Camino Francés at Astorga and walk along the northern branch of the Vía de la Plata toward the south, to Granja de Moreruela. There you switch to the western branch alternative of the Vía de la Plata, via Ourense to Santiago. The landscapes are just as beautiful as along the Camino Frances, there is an excellent network of albergues, and ... the route is much less crowded.
Leave the Camino Francés at Ponferrada and walk along the Camino de Invierno, via Lalin, to Santiago. This route was also used in former days, especially in winter (Spanish: "Invierno"), to avoid the snow on O Cebreiro.