In the history of Pokemon games, Zygarde seems to be the weakest legendary creature of all. However, this won’t be the case in the future Pokemon Z game.
Fans are behaving rather loudly in calling for Zygarde to be much stronger and play a much bigger role in Pokemon Z. This can only be achieved if Nintendo creates Mega Zygarde.
In the current X & Y, both Xernias and Yveltal are immune to Zygarde’s main types. A Mega Zygared should be able to reverse that order and the immunities will turn to weaknesses.
Having a Mega Zygarde will automatically add 100 points to its base stats, hence, putting it in the same platform as Mega Tyranitar and Mega Garchmop.
As for the plot, the fans are quite creative with their ideas. Most fans agree that it should have something to do with the Pokemon ecosystem which might go wrong and Zygarde is the key to fix it. After all all, Xernias is like a tree, Yveltal is like a coccona and Zygarde is like a larvae. All three co-exist with each other.
Of course, Nintendo must have their own ideas up their sleeves. Nevertheless, there is no denying that Zygarde needs more forms. All of this might just happen in the form of a DLC or even Pokemon Z itself.