OK, I'm a new member, it took me awhile to read through this thread but get ready for another Adolinista. I can see Adolin remaining a secondary character but please don't go all GRRM on him. I just hate that.
So maybe Adolin eventually gets an Honor blade and as Shallon's mate becomes her champion. She is the main character and not suited to fast fighting action but has him as her protector so that she and Pattern can work their magic during a battle. Another possibility, Shallon works her magic and awakens his blade by drawing a new spren for it. It just seems he would be her very dedicated knight and having her give him the emotional support he seems to need would fill out something for both of them.