Backstory on Silver would be great, but nothing that'll end up changing the main plot, because it really isn't something that needs to be changed. If they give some people a mention of him and gradually tell of his history, then I think that'll work. With the new ecnounter possibly later on with his newer evolved Pokémon, then maybe they can add in something where he himself explains the story as well for somethings that people may have missed.
Something I was thinking of though is his appearance. He is given a tough look for every battle we've had with him, but at the end he changes. With this change, and maybe the extra fight they may add in, I say an alteration of his clothing design can be touched up on as well. He is one of the best rivals they've made for game play, and if they can simply show his character change with a new style afterwards I think it'll add on as a cool twist. They did it with Gary in FRLG after he became champion with his new pants, and seeing as how a DS cartridge can do more, I think something as that for Silver wouldn't be too difficult.