The end of the novel is less a conclusion than a beginning. Really, it's almost as if the entire book is a preface; the explanation of the gigantic detonation which was required to get Ignatius out of his house and New Orleans and off to the rest of his life. "[N]ow that Fortuna had saved him from one cycle, where would she spin him now?" Ignatius wonders, adding that, "The new cycle would be so different from anything he had ever known" (14.132). Considering that he's never left the city before, this seems likely to be true.
Ignatius's uncertainty here is matched by the reader's. What is Ignatius going to do now? What will his life be like, separated from his mother, separated from his house, separated from New Orleans, living with Myrna in New York? Will he quickly revert to his "old, horrible self" (14.129), as Myrna for a moment suspects? Or will a new place make him a new person?
His valve opens as they get onto the highway— but is this a sign of renewal? When he presses Myrna's pigtail to his wet moustache in the last line, is it actually a sign that he's actually grateful to her, which would be perhaps the first time he's been grateful to anyone in the entire novel? We know that he's been obsessed with Myrna for basically the whole book without admitting it, so what happens now that they're actually together? The novel doesn't tell us; it just runs out of plot on the borders of New Orleans.
Toole's original publisher wanted changes in the book because they felt that it was not meaningful enough, and this ambiguous, whoops-we're-done-now ending must have been one of the things they disliked. But the close suits the book, which is less a sustained story than a bumpy stream of incidents, cleverly knotted together by chance and the general rule that if anything can go wrong, it will.
You can't expect profound meaning or neat conclusions from a confederacy of dunces, after all, so the only thing we can say for certain about Ignatius's future is that, if Toole imagined it, it was probably funny.