I'd combine Half-Life 3 and Portal 3. Chell ends up meeting with Gordon. The Resistance hacks into the Combine stuff and finds out that they are going to be launching a huge attack. They also learn that GlaDOS has a special core that scientists developed in case this exact scenario happened. Unfortunately, the Combine know this and have taken over GlaDOS. Gordon and Chell fight through Aperture Science. They find GlaDOS and fill her in on what's happening. The core kicks in, but it doesn't have enough processing power to scan the Combine plan and find weak points in time. Then...Chell volunteers for her consciousness to be uploaded to GlaDOS. Their combined intelligence breaks through the Combine plan in minutes, sending their own weapons against them and destroying them.
Then, G-Man congratulates Gordon on a job well done. Fade to black.
And then? And then, you ask?
Ending theme: Giggle at the Ghostie
