So I'm curious what people want from boss fights. Because at least on spectacle, the ending of ZA delivered IMO. Now, this is partially that I consider the entire "destroyed Lumiose" traversal section to be part of it, but even just the fight involved big, flashy moments while also needing a solid amount of skill from the player. (Not a ton of skill, it's still Pokemon and the level curve in the game isn't great).
Personally, I think the bosses throughout the entire game were really well done. For a Pokémon game, there’s plenty to worry about like AoEs, unique mechanics like Dragonites Hyper beam, Banettes shadow clones, Frosslass’ arena attack, and Rayquazas tornados.
Really it’s just Rogue Ange who’s the boring boss fight to actually fight.
It’s got multiple phases, but they’re all functionally the same, but with one flower gaining an AoE attack around itself.
With the rare meteor shower attack Zygarde protects you from.
Personally, here’s what I would have done for Rogue Ange:
Phase 1 works like it already does, you have two flowers, you need to beat them.
In addition, as the fight progresses they gain more attacks to worry about, such as sweeping lasers you need to dodge or take cover to not get hit, vines that can attack you from melee range and an increase in numbers, etc. maybe have a building be thrown at you if you stay too far away.
Phase 2 changes the main target, now you need to attack Rogue Anges legs to keep it from moving and help Zygarde. The two flowers are still there but with significantly less HP, but can also move to different spots and you need to worry about them. Taking one out will cause it to leave for a while, but eventually it’ll return.
Phase 3 has you attack the other leg. Now with mini flowers on the Leg trying to fire on you, alongside more Big flowers.
Throughout the fight the other characters can also show up with their Megas to help draw the flowers attention away from you for a time. As a ‘hey you’re struggling so here’s some help’ type of thing.
And the invincible barricades can be destroyed after enough hits so you can’t just use something like Mega Meganium with Giga Drain and hide behind a rock and not worry about anything