Idea One:

As  ladyyueh pointed out in 'Doctors with Accents", Chase looks an awful lot like Five. Once that connection is made, it isn't hard for your mind to go down the following path;

Tegan is Australian.
Chase is Australian.
Tegan travelled with Five (and seems to be the favorite target for Five related pairings).
Chase looks a lot like Five.

It's pretty obvious where I'm going with this. Admittedly there are problems with this idea, the dates don't add up and there are bit of House canon which clash with the idea but both of these could probably be fixed with a bit of creativity. There's all sorts of ways you could play it as well. 

Idea Two:

 Credit for this one goes to box_in_the_box, who came up with the idea as part of their personal fanon. Since the idea was quite frankly brilliant but neither of us was up to writing it, they said it was okay for me to post it here.

1. The heretofore unmentioned whereabouts of Romana will be revealed in a crossover with House, when Dr. Lisa Cuddy finally opens a mysterious fob watch that she's been carrying around all this time ...

Told you it was brilliant.

~Weaver