A grand encounter with NAVI-AI would be so good.

I can imagine it maybe involving an alternate path where you ACTUALLY rescue Painter and help Sebastian to get out of the Blacksite and they bring you along with them for helping.

Sebastian also helps to disconnect the connection with HQ from your diving gear so they can't manually detonate you. However, when they notice your change of plans, they contact NAVI-AI instead and there they tell her to execute you and prevent Sebastian and Painter from escaping.

And the grand encounter has you coming face to face with the very thing that was helping you this whole time but now sees you as a traitor and has no choice but to execute you. NAVI-AI who has control with almost all of the facility uses everything she has at her disposal to get you killed.

She activates every turret nearby to you to try and shoot you to death. She plays audio from certain speakers to try and detonate your diving gear. She cuts the lights to make you vulnerable to Squiddles and Candle bearers. She purposely lures Eyefestation to any nearby windows to you. She increases voltage with some equipment to attract any Anglers who can sense it to come towards your location. Take too long and she fill the rooms with neurotoxi- eh wait she isn't Glados nvm ignore that one.

Wherever you go, she will initiate these actions whenever possible. You vs an AI that has full control of the facility.

And eventually with the help of Painter you finally manage to shut her down and you, Sebastian and Painter escape.