I own none of the IPs represented in this collection of ramblings. If I did, I wouldn't be attending school.
This is my take on the "Harry gets an insanely magical trunk" plot device.
An eleven year old Harry Potter peeked his had around the corner of the, extremely well protected, Potter family vault and looked at the solitary piece of Luggage sitting there. He didn't know why, but for some reason calling it "luggage" wouldn't have been appropriate, it demanded the capital "L".
He glanced back at the absolutely terrified Goblin cowering in the cart behind him. "Griphook, shouldn't there be more in there?"
The Goblin flinched at hearing his name and huddled a little lower "No!" came the high pitch reply "No, that's it! That's all that's ever been in the Potter vault!"
Harry turned back and immediately jumped backwards. Sitting right in front of him, innocent as can be, was the Luggage. The Luggage that had, until he'd turned around, been a dozen meters back inside the vault.
"Well," Harry mused "if you're part of my family's estate then you must be important, mustn't you?"
He vaguely wondered if he should be feeling foolish for talking to a perfectly normal, if, for some reason, highly intimidating looking trunk. For the life of him, he couldn't shake off the feeling that it completely understood what he was saying.
"So," the new student continued, "can you follow me out of here?"
In response dozens of tiny legs sprouted from the underside of the trunk, and Harry got the distinct feeling that this is what having an exceptionally intelligent dog would be like. Only something told him no dog could ever match what the Luggage promised.
The terrified squeal that came from the bottom of the mine cart drove that home.
So, the Luggage comes to Hogwarts and pretty much runs roughshod over all of JKR's convoluted plot points and Voldemort's mustache twirling style evil schemes. Terrorizing Snape and Draco are a must. Dumbledore can be either an evil overlord (which frankly. I'm getting tired of seeing, but its up to the author)_ or an old man who has had the weight of the world on his shoulders for a inordinately long amount of time.
The Luggage in this story is the oldest heirloom of the Potter family, dating back to their origins in the founding of the Roman Empire, to and unbroken line of descendants all the way to the modern day Potters (suck on that, Malfoys). It stores the entirety of the Potter Fortune that isn't in Harry's trust vault, and only coughs up gold for paying fees and services (which is magically transferred to the Potter's pocket/money bag/whatever), all money earned by the Potter family is magically transported into its limitless depths. If anyone wants to take this and run with it, I enthusiastically encourage you to do so.
If no one knows what The Luggage is then google the Discworld wiki and look it up. You barbarian.