My first inclination would be to seize upon the meaning of that last line and try to turn it a bit. Perhaps Rick is being sly when saying this -- knowing it will now be easier to double-cross or use Louis's friendship to his advantage. Or perhaps he is being sarcastic because he knows that Louis's real intentions are to make life difficult for Rick be always keeping an eye on him.

For me, it would have to be about the friendship:

They arrive in Brazzaville, set up an operation together, and then something threatens the friendship. Maybe Louis is forced to betray Rick because his wife and daughter's lives are on the line. Complicating this, Ilsa shows up. The plane was forced down before clearing Morrocan airspace, and her husband was taken back to Casablanca. Now she's returned to find Rick to help her get him back. Rick has to choose. Eventually he finds out the two events are connected. He slips back into Casablanca and becomes a detective of sorts, searching out Ilsa's husband and Louis's family, held in secret while they (the Nazis) wait for Rick and Louis. Why? Because Louis is actually working for the Resistance, and knows the identity of the leader who is so effective against the Nazis. So, Rick and Louis have to rescue Ilsa's husband and save Louis's family. So Rick organizes all the street hustlers and thugs from the Casablanca underworld to get it done, ending in a total defeat of the Nazis just as the Americans hit North Africa for Operation Torch, and the eventual confrontation with Rommel.

Okay, I know it tramples on everyone else's imaginings for Casablanca, but don't you think it has the potential to be a great sequel in its own right? If anyone out there is interested, contact Lee at The Last Reveal: http://thelastreveal.blogspot.com/

Let's not dismiss the homosexual undertones of the relationship. This really is the beginning of something now that Ilsa, the intruding obstacle, has been cast out of the way.

The true sacrifice in Casablanca is the one made by Ilsa, who was well aware of Rick's closet conundrum.

A sequel? Rick and Louis open an underground S&M bar called "Dick's" and become pioneers of Brazzaville's burgeoning LGBT district, which infuriates the ethnic Bantu, and ultimately leads to their fatal lashing.

They talk about a sequel yet a prequel could be an option? Everyone knew the ending of the titanic. Everyone knows the ending with Casablanca.... And, there is back story of Paris in Casablanca.