Apologies for the essay (I should get back to my college ones, but there ya go)....
Original Doll was an album that Britney was supposed to release around 2005-2006 and it was meant to be her fifth album after In The Zone. Britney took a demo of Mona Lisa (meant to be the first single from that album) to a US radio station without her label's permission and asked them to play it. In an interview around that time she said that she was working on her album that was "likely to be titled Original Doll" but for some reason the album was never released. It's likely because she had her kids around that time, but there are a lot of conspiracy theories on the 'net about it. Some people believe it was to do with the Illuminati and that the album was destroyed because it was supposed to be a "personal" and raw album and the powers that be didn't like that idea, but that might be a bit Farfetch'd
The closest we got to Original Doll was the Chaotic EP (which has Mona Lisa on it). Some songs that were intended for the album leaked over the years (Peep Show, Ouch, Conscious, Money Love Happiness, Welcome to Me and others I can't remember) but nobody knows what happened to the full album. It's one of the biggest Britney mysteries out there, along with another unreleased song called Rebellion. Only a snippet ever leaked and the full song is out there somewhere but nobody's leaked it. An official tracklisting never materialised and there are tracklistings online but they're fanmade. Some fans have made albums/playlists of songs they thought were intended for it.
"Can You Handle Mine" is likely a fanmade album. Some thought it was Britney's fifth album after ITZ and the title comes from a line at the beginning of My Prerogative. I looked it up a few months ago though and it's actually a fanmade album of random unreleased songs. If you're looking for unreleased Britney songs though it's worth checking out.
Abroad is an unreleased song from Circus and also a fanmade Britney album with unreleased songs on it, including the song Abroad. I downloaded the album back in 2012. It was the first time I had ever heard any unreleased Britney songs. IMO Abroad (the song) should have been put on Circus instead of My Baby (while I understand and appreciate the sentiment of the song, I don't understand why it was on the album and I think it should have been a free download as a gift to the fans) or even instead of Radar.
I'm quite fascinated by unreleased Britney music and I know a lot about it