Will an Ash-Greninja Bond Phenomenon be back in Pokémon Legends: Z-A? Why?

No. Because Ash isn't the protagonist anymore, and the player being able to turn their Greninja partially into someone they don't know (as the player probably isn't Ash) through their "bond" is weird and does not make sense. Plus there weren't abilities in PLA and it will probably be the same in PLZ-A.

AND although it looked cool, the bond phenomenon was horrible in-game (and also a bit in the anime). It further decreased the popularity of Delphox and Chesnaught, gave Greninja a form stronger than a mega while giving the other two less than nothing and a connection to Ash, plus it was kind of a stupid idea in general to make the BOND PHENOMENON Ash Greninja ver. (We don't know if other versions exist yet), which is supposed to have made Greninja look closer to the person it bonded with (Ash in this case), available to all people?!

To be honest I just hate Greninja (and the XY anime) for contributing to making the gap of popularity between Greninja and the other two Kalos starters somehow worse than Charizard and the other two Kanto starters. TPC just can't stop favouring one starter whenever they even think of Kanto, huh. At least Venusaur and Blastoise got megas and are part of the original 151.

Also it's just an awful excuse to give more attention to Greninja than the other two, since we've never gotten a new bond Pokémon. It was just a singular form for a singular Pokémon.

"It was so extremely close to Ash, it created a new form change even stronger than a Mega. But because the sheer strength of their bond is so great it was only achieved once in the entirety of Poké-history this form probably can't be replicated ever again! What a convenient excuse to only give one of the starters a new form while the others get hated on for dumb reasons and stay obscure and liked by few forever, haha!"