As far as we know, illusion may only change Zorua/Zoroark's appearance. This would make it far from a ditto/random character. Since we still don't know very many of the new pokemon yet, I don't think we can make a very good guess about what new pokemon will be added. Zoroark is the most popular new pokemon right now, but remember he started getting votes before any 5th gen competition even appeared, so whatever he is, he'd probably be in the lead there. Still he does seem close to a pokemon who'd come next in a pattern that starts with Mewtwo and then Lucario.
As for old reps, Pikachu and Jiggs are obviously in. Lucario is probably out, unless they decide to try to get a rep from each generation, which they did try to do in Brawl, if not for the cut of Pichu and Plusle and Minun. If they go for at least one rep per generation, a guess would be making PT have 2nd gen pokemon (please not Pichu!), Plusle and Minun for 3 gen, Lucario for 4th gen, and Zoroark for 5th gen. On the other hand, more people would probably rather see the lineup including Mewtwo, PT, Plusle and Minun, and some new pokemon, which at the time is apparently Zoroark. Most other pokemon like Meowth have really low chances of getting in, unfortunately.
However, there's still one old gen that's sort of a hole, and has a chance of getting an all new rep, and that's second gen, if Pichu does not return. Some possibilities for this are a PT, Togepi, Bellossom, Marill, Unown, Wobbuffet, or Scizor. I admit none of them have good chances, but all have been a pokeball pokemon in at least one game, and hey, were Plusle and Minun really that popular? I would pick either PT, Bellossom (if Ivysaur is gone), or Scizor, as all would be great additions.