Great Idea. As it is now, Naples and Sicily are rarely conquered up until the Empires are so huge and powerful. Spain is the one who usually takes Naples (as I guess it's still a OK province to them in 1.0, but I didn't check), and Sicily may hold its position until the end of the game, if no one is brave enough to take a huge amount of troops to conquer it.
In my games Italy is always a target for expansion for two reasons: Venice and Genoa are usually weak (at least up to 1500 when, if one of them survived, it should be powerful) and Florentia and Naples are always independent and no one wants them (only in the case of Genoa or even Venice sometimes, but it ends up in the first reason). Allied to that, Italy is easy to protect (and that's why no one wants it, or no one wants it and because of that it's easy to defend, or both, or not, but it's easy to keep it anyway).
A Kingdom of Sicily seems nice to fill that area and make it more challenged.