Honestly, let's see some reasons just listed of why it will be so good.
1. It hasn't been on Napoleonic Wars. There was a faction replacement mod, but it never was added in as a faction in the DLC.
2. Good-looking units with a lot of variety and a lot of information about them.
3. It will be something new compared to the already massively known Prussia, Austria and Russia. This could be a nice change.
4. Two-sided history on the conflict. You had Spain that fought against the French and a Bonapartist Spain that fought alongside them.
5. It fits the setting of the uniforms in the game. It was confirmed they are set for 1809 and during that period of the war, well the Peninsular War raged on.
6. Spanish Navy. Yes, it would be more interesting to see, if it could take a part in the naval aspect of Holdfast.
7. Expansion towards the Spanish community. Well, I'm sure they as well wanna play their own nation as most people do.
Oh yes, variety is what makes this suggestion godly, and seemingly a lot of people are up for it, so why not do it?
I hope it is added in after UK and France. Only that I couldn't change my vote to that after I started thinking what good it would make for the game.