In this thread, it was clarified that the epic battle with the Guardian of Ukaizo is intended to only take place in response to certain past decisions by the player. (And I also learned that a fight can be avoided through dialogue if the player is an Island Aumaua!)
I wanted to follow up on my response in that thread, with an idea that I hope is consistent with the devs' design, but guarantees that all players get to experience an epic final battle:
1. The initial battle with the Guardian may not take place, or can be avoided through dialogue. That part is unchanged.
2. But, like in Mass Effect 1, there is a surprise, second unavoidable battle with the true final boss.
- Upon leaving Ondra's Spire and entering the Grand Promenade, and defeating the faction leader, the player finds the path to Eothas blocked by a mystic force/barrier. The Watcher can sense that the source of this barrier traces back down to Ukaizo Harbour. At this point the various party members give their "before the final battle" speech.
- The Watcher enters Ukaizo Harbour...
(If the player did not encounter the Guardian, a sparking, injured Guardian stumbles out of the water and speaks with the Watcher like before.)
The Guardian's body, ruined or otherwise, rises into the air and you sense Eothas' presence in your mind.
"This statue of Maros Nua has witnessed many secrets, Watcher of Caed Nua. Indeed, there was another being who thought herself Master of that great keep..." The dialogue reflects whether Watcher encountered/aided/defeated/never met the Master Below in Pillars 1.
- The adra pillars jutting into the harbour being to dissolve; soon it is a blizzard of adra flakes. The flakes of adra adhere themselves to the body of the Guardian, in the same style as the Adra Dragon (the Master Below) from the first game. The idea is that the adra flakes heal any injuries and act as armoured scales for the Guardian.
- This "reborn" Guardian (Eothas' portfolio is rebirth) is an adra-encrusted version of the Guardian of Ukaizo in the same manner as the Adra Dragon. It has a sliver of Eothas within it, causing its forehead to bear his glowing sigil, like the statue of Maros in the introduction. It may not be a battle with the giant statue, but it is still a battle with Eothas in a sense.
- Upon defeating this reborn Guardian, the mystic force barring access to Eothas is dissolved and the player can proceed to the engame.
In a way, this battle would come full circle -- the Adra Dragon lay at the feet of the statue for so long, and invokes the memory of the Watcher's struggle for Caed Nua.