I think it might take place in the past, but i dont think thats roxas, it might be a former keyblade master because if you noticed when they show king mickey at the end he has the keyblade that yen cid gave sora. I think it might have taken place when micky was still under the teachings of yen cid.... also in KH1 in the letter mickey already knew about the key blades and yuffie said the key blade chooses it's master so there has been others before sora, maybe they just all look like sora and roxas?