First, let me say I'm glad you guys are creative about this stuff, it's great...but there are some issues that need to be addressed in regards to this:
#1: Your character cannot have an "uber" background. What this means is that you cannot have a backstory where your character is the son of a god, a king of some powerful kingdom, related to a "famous" NPC like Elminster, etc.
#2: No gods or demon lords or similarly powerful divine beings showing up. This just isn't possible in Ravenloft after Vecna nearly destroyed the demiplane--the Dark Powers won't allow it to happen anymore. I have heard some things that concern me like Ezra "showing up in mortal form" (totally impossible) or PC's possessed by well-known demon lords like Graz'zt or Demogorgon (unlikely due to their interests in the outer planes and their knowledge of Ravenloft as a "one-way trip"). Sorry folks, but you should check with a DM first to see if this is even possible before working it into your PC's story.
#3: Your PC cannot be the hero from another D&D video game like Icewind Dale or the NWN official campaign. It's debatable whether this stuff is even canon to begin with, and what happens when two people decide they're the main hero from Baldur's Gate? To avoid this kind of conflict, come up with an original background for your PC.
#4: Your PC cannot just be spontaneously possessed by a demon, etc. Things like this would normally happen in the course of a campaign at a DM's discretion, not be in the past of a level 1 n00b PC. You may propose a story like this to the DM team, but be prepared for possible changes to fit the canon of our setting. If it is approved, it will occur as a plot that is run on the server and not be your PC's backstory from day 1.
As long as you follow the above guidelines, we welcome and encourage you to come up with your own stories here on PotM.