Traveling in Spain for more than three weeks will be a wonderful experience!
Well, it is great that you are starting so far in advance, because you will have ample time to plan, research, and finalize details. So far you have a very nice route to the middle, South and East of the country, with sufficient days in each location to really relax a bit as well as sightsee. I dislike packing and unpacking every night, so try to avoid too many short stays.
Now you have time to consider the logistics--what is the overall budget, what type of hotels do you like, have you started looking at train schedules this year to see how they might affect your arrangements for next year, do you completely understand what is involved in the Feria de Sevilla, and perhaps you will also be in Spain during Easter, so can start researching that as well. Most importantly, what exactly do you want to see and do in each of the places on your itinerary?
Okay, when I travel I always have very, very specific things I must see and do, even including what restaurants I will be reserving! Foodies will love the diverse dishes and wines of Spain, and trying tapas is quite enjoyable. Always save room for chocolate and churros in the morning if you dance in the nightclub late at night!
In Spain husband and I are enraptured by history--our priority is to visit castles, cathedrals, walled medieval towns, Roman ruins, Moorish heritage architecture, and so forth. We will squeeze in a few beach days as well as ample time for shopping. We often stay in the paradores or historic hotels. We have traveled by train as well as renting a car, or short flights, and when I was much younger, I even hitchhiked around Spain.
Religion has a mighty heritage in Spain--yes, I am always moved by the magnificent cathedral of Santiago de Campostela, but small chapels also contain deep spirituality.
The literature of Spain can also influence travel decisions-- Cantar de Mio Cid, Don Quixote, Don Juan, are all woven in Spanish culture. Of course, the music and dance is phenomenal-- I have formally studied flamenco for multiple years, but the experts blow us all away with their syncopated moves!
Well, Spain was always known for bull-fighting, so that is always an option. Nowadays soccer (fútbol) is one of the most popular sports.
Some of the greatest artists in the world resided in Spain, and you can visit their masterpieces in many museums.
I am going to close with languages--Spain is more diverse that visitors understand completely, and many of the regions have their own unique culture and language, although most are derived from Latin, except for the Basque area.
So take some time to learn and feel the country that you want to explore, so that your visit becomes more than just a tourist bus stop and start, and instead you create deep meaning and start to uncover the passion and pride of the people of Spain for their wonderful nation.
¡Buen viaje!