This seems like an odd coincidence. From this website I read that Silver is based on Trunks from Dragonball Z.
1. Both are from the future and try to fix something in the past.
2.Both lose someone they care for (Silver: Blaze Trunks: Gohan)
3. both learn something that helped them defeat a disaster
Don't belive me? http://sonic.wikia.com/wiki/Silver_the_Hedgehog Read the first paragraph.
And to me, Shadow and Vegeta, also from Dragonball Z, are similar
1. Both have unusual/rare bloodlines (Vegeta is royalty. Shadow has BlackDoom's blood)
2. Both are confident, powerful, strong, and cocky (For Shadow, before he lost his memory)
3. At first, both of them are villans to the main characters, but later become allies, but still keep their distance.
4. both have spiky, black hair (black quills for Shadow)
And yet, from this same website, I heard that fans first thought that Silver was Shadow's desendant, due to similar features. In Sonic and the Black Knight, both of them are in the game, but are really knights of the round table there. Silver is Sir Galahad, and Shadow is Sir Lancelot. In The Arthurian Legend, Sir Galahad is Sir Lancelot's son.
Again, you can find the proof in the link above. Look under Trivia at the bottom of the page to find it.
Then, Vegeta and Trunks are actual father and son, (My brother knows a lot about the show, and that is what he has told me, and belive me, he doesn't lie about this stuff) and being similar to Shadow and Silver....This is really werid.