Version numbers keep track of development, but they aren't the same thing as mathematical numbers. Though there are some general rules that are followed, the numbers are arbitrary. For instance, the release after 0.9.2 wasn't 0.9.3 or 1.0.0, it was 0.10.0. The general rule here is that the first number denotes whether the game is complete. There can still be bug fixes or expansions after 1.0.0, but generally the game is finished. The second number is the current release and increments upwards with each new content release. The third number is for bug fixes. It's possible to have enough bug fix releases to go from 0.37.9 up to 0.37.10 or something of that nature, but it's unlikely. It's also highly unlikely that the game will reach 0.60.0 before development is complete.
As to the matter of expansions, Sierra has stated that she might consider a spinoff in the same world, but it is unlikely. Also no direct sequel or continuation of the story with the same characters is being considered. The story is planned from the beginning with a definite conclusion in mind.