I'd say it's possible, but not necessarily likely... There's no guarantee such a set would have LEGENDs or Primes, though it's a safe assumption. Whether they're new or reprint is anyone's guess, though Power Keepers' EX's were all new, so it's possible. (There are a lot of possible Primes/LEGENDs. If I had to make a prediction, I'd say one of the other trios made in the style of the Johto trio's LEGENDs, and some unpredictable Primes. I'd have to wonder, though, which region the set would presumably be based on... I'm doubting Johto would be, since it's already been a focus, and all its logical Primes/LEGENDs are done. Kanto seems the best bet. If that were the case, I'd hazard a guess that it would be Articuno/Zapdos/Moltres LEGENDs and Venusaur/Charizard/Blastoise Primes, not sure what, if any, others would be included, maybe Mewtwo or Dragonite...)
Of course, all that speculation is only if there IS a filler set. Again, in no way are there any guarantees. Still, I hope there is one. Reprints would be nice, especially if they're of older cards that aren't currently legal.