While I think its unlikely, I with you guys in that I really like the idea of mystery plane person being Alexis.
The MC is kind of in a fresh state. Because you've either closed the Alexis route, or you've kept it open but she's rejected the relationship for now, and Cara/Nikki are either not a thing or in a bad state after The Punchening. And you're either flirting with Erynn, but not in a relationship, but also kind of mad at her over Douchebag McGee. And you've maybe kissed Layla.
So outside of Layla, the MC is sort of in a similar position as he was at the start of the game. It would be a nice narrative loop and callback to the start of the game to have Alexis be your passenger buddy again, and this time (whether you're in a relationship or not) start anew without any secrets this time.
Still think its Lakeyn, though.
My concern with that is that it feels too much revolves around Lexi.
We still have a few unanswered questions and unknowns after the end of v0.6.In no particular order;
-What is Erynn's secret job? I reckon those that have suggested it is either escort or possibly porn are on the right lines.
-Who is Erynn's father?
-What is MC's father Santiago currently up to?
-Who was that sports agent the MC met in Dylan's bar - he MUST be connected to somebody?
-Is anything further going to happen between MC and Gabi?
-Could anything happen between MC ahd Elena? She has been flirty with MC but presumably would be loyal to Layla?
-What is the deal with Sienna? She has cropped up a few times so far - is she just a backing character?
Add in Principal Moore's reaction to the MC dating both her daughters. And getting some resolution to the MC / Lexi / Erynn triangle and the MC / Lexi / Layla triangle, and there is still the potential for much drama to come!