To be honest, there have only been two major relevant Android versions; 2.x and 4.x. Under 4.x, they realised that getting updates to devices (where they had to rely on phone manufacturers) just wasn't dependable, and they overhauled both their OS's architecture (moving most services out of the core OS into Google Play Services) and their release system (allowing for over-the-air updates without the phone manufacturers' intervention).

So unless they've rebuilt Android from the ground up, I doubt they're going to do a 5.0.