Back to BTTF, for a moment.

It's funny. BTTF2 & 3 sure form a matched set, somewhat like ESB and RotJ or Matrix 2&3, PotC 2&3etc... But I think BTTF 1 fits in less with its sequels to form a "Trilogy" (capital 'T') than Star Wars or the others.

Star Wars sets up the Empire, that the rest of the trilogy must deal with.
Matrix sets up the... Matrix, that the rest of the trilogy must deal with.
Chronicles of Riddick sets up several things that the rest of its.... *boo hoo*... [uncontrollable sobbing]

Back to the Future, joke or not, sets up it's sequel at the very end, including one of the only movies to be first in its franchise and yet promise a sequel without knowing how well opening night would perform. And yet, 2&3 are about problems (the almanac, the west, Marty not liking being called a chicken) that are in no way connected the first movie.

Unless the Trilogy is called "the perils of time travel" and said perils include:

accidentally negating your own conception!
Trying to get rich by taking information to the past about the future and accidentally making the biggest butthead of them all get too powerful
Rabbits full of buckshot.

So... it's just funny that it ends with "to be continued" but it has less to continue than most other capital T Trilogies. Star Wars, Matrix, etc.. were definitely instances where the sequels were put into production after the success of the initial movie, but where there was a problem that was still left to be resolved. Those are tight Trilogies. BTTF a little less so. PotC a little less so.

Funny. Ha ha.