One of my favorite characters in recent memory was the Princess Venyra Salitzar. She wasn't a missing princess: Everyone knew she was going on the adventure to the strange continent that appeared in the middle of the ocean because her cousin was the person organizing and leading the expedition.I'm always open to negotiation, but in general we don't use much 3PP unless it's on DDB, and I don't allow aarakokra as a starting species. Plus we have a few house rules, and I ignore RAW that hampers the game experience we want. I also don't use WotC settings like Eberron and Planescape. And alignment doesn't exist in my games. Also, pisionics. I hate psionics and try to avoid them.
Edit: also, certain character backgrounds that hijack the story in a super cliched way. Nobody gets to be secretly the missing prince or whatnot. If you want to tell that story, you DM.
The -party- didn't know she was a princess, at first. And kept sarcastically referring to her as a princess 'cause I played her as a character of noble birth and stuff with the three servants the noble background affords.
It was hilarious when they found out she -was- a princess. And a little heartbreaking when she found out they were calling her princess as an insult. Since by that point the group had actually become fairly close friends.