I don't know what could have possibly been more interesting than Bitcoin to move onto, but my favorite (conspiracy) theory is that he "moved on" to the development of Namecoin, the first real altcoin.
Why leave the main project (without closing this project) and move on to developing a secondary one (the creation of which implies that this project is better, otherwise why create it at all)?Mind you there is no explicit evidence that satoshi was actually involved in the development of Namecoin, or that he and Vincent Durham (thought to be a pseudonym), the first developer of Namecoin, are the same person.
Maybe the whole point is that Satoshi has nothing to do with this at all? 
I think it's an honor to have such a conversation with satoshi, not just in email, telling that he's moved to other things and Bitcoin is in good hands with Gavin & everyone.
Other things could be anything. Even a small cafe on the seashore. Why did everyone decide that Satoshi had definitely moved on to create a new crypto-coin or a project directly related to cryptocurrencies?The most likely (in my opinion) reason was that Satoshi's remaining time was limited and he
). At least, it would have been a beautiful legend (like an action movie hero walking off into the sunset after defeating all his enemies ) justifying his decision to leave his project.