I went to look at the TCG cards for the starters cause I was wondering if they leave hints to their z-moves, if they get any that is. And I know the TCG shouldn't be the first place to look for hints of pokemon info, but I know some GX moves are actual canon z-moves like Snorlax. So can't hurt to speculate.
Decidueye: Hollow Hunt GX: Put 3 cards from your discard pile into your hand. So maybe a reviving move.
Incineroar: Burn Slam GX: 200 damage. The Defending Pokemon is now Burned. So basically deal heavy damage, and if they survive they get burned as punishment.
Primarina: Grand Echo GX: Heal all damage from each of your Pokemon. So a healing move. Kind of expected.
So how could these work as z-moves in the game?
If i'm not mistaken, Snorlax' Z-Move requires it to have Giga Impact, right? The starter's Z-Move could work similarly, but with their signature moves, being improved versions of them.
Decidueye (Hollow Hunt): the name actually kinda fits. It could be a volley of arrows instead of a single one, hitting all the opposing field. It could also either be a multi-strike move to get past substitute and stuff or always crit. Oh, and apply the trapping effect.
Incineroar (Burn Slam): Stronger version of Darkest Lariat. Single target, huge nuke, ignore stat-changes, and always burns. Might have recoil as well, possibly. Weird name for a Dark move, but whatever, it could be a wrestling move.
Primarina (Grand Echo): Damage to the opposing side (either sound-based damage, or play with its flavor text, making it control a barrage of bubbles with its voice, making them explode on the opposing side). Might heal the ally side at the same time, either HP or status conditions.
About Hidden Abilities, for less broken option then Serene Grace, Primarina could get Marvel Scale (the one Milotic has, not sure if that's the name). With that awesome typing and good bulk, could throw a good Restalk set. For Decidueye and Incineroar, the most obvious ones have already been said. Maybe Cursed Body, if they want to go the Ghost way.