Well Xerneas has a type advantage over one type of Yveltal and this pokemon could still hit both with atleast one type. I'm just saying guys there's still a possibility of a Z legend.
One of them have always had an advantage over another though.
Lugia learned a multitude of Water attacks which gave it the advantage over Ho-oh
Kyogre had Type advantage over Groudon.
Dialga had Type resistance over Palkia (Steel vs Water)
Reshiram and Zekrom were the first without any distinct advantage over one another.
Xerneas has type advantage over Yveltal.
Only 3 of these have had Third legends associate with the others.
Rayquaza resisted Kyogre's Water and was immune to Groudon's Ground (also resisted his Fire though he wasn't a fire type)
Giratina had no outward advantage over Dialga or Palkia. Though Dialga resisted Giratina's Ghost type. (though not anymore =D )
Kyurem and Reshiram were neural to one another due to resistance and weakness checks while Kyurem only had advantage over Zekrom due to Dragon being weak to Ice. (Both were superior to Kyurem stat wise though)
This new Z is inferior to both Xerneas and Yveltal as both are immune to at least one if his types. He'd have to be a crapton stronger than both like...near Mewtwo level strong to make up for this.
Also there is a rumor saying Mewtwo is #150 on the Mountain Dex taking the Regional slot that Z should have which could lead to Z not existing at all as well. We could end with just 717...or we could have a Sinnoh pulled on us and a few more Evolution/Pre-evolutions added after the main legends then lead into another set of legends (the triplets) and the final legend/Event Legend.
We have a lot of past things to go on. And almost no way of identifying any possible events either so we don't know how many if any events are in this game.