Okay, so everyone knows how Sirius Black died in HP, but maybe he didn't die. Here's my theory:

After Sirius fell behind the curtain, he was sort of in a state between life and death. Lucky for him, he finds a door. He opens the door and finds himself in the Room of Requirement, thanks to the many mysteries of Hogwarts. He soon finds the passage way to Hogsmeade, and apparates, since he's not on school grounds anymore. He goes to Grimmauld Place and gets rid of the death eaters. After that, Sirius says to himself: I've gotta tell Harry I'm alive! But, he'll get distracted, so I'll do it later. Then Sirius reads the Tales of Beetle the Bard, and says: I bet Dumby gave Harry the Ressurection Stone! When he turns it thrice in palm, he'll expect me there! So, he goes to Hogwarts at the battle, and casts a complicated patronus charm thingy and makes it look like a Sirius ghost. He stands behind a tree and makes it talk. Afterwords, he fights some death eaters, and goes home. After the 7th book, Harry and Ginny who are now engaged, go to Grimmauld Place because it's Harry's house now that Sirius is (sniff sniff) gone. They arrive only to find none other than...Sirius! Alive! And they live happily together as a family. Yay!