Apple has released 5 versions of the iPad since the '1' was introduced in April 2010. Add the regular and the retina Mini iPads and you've got seven versions in four years.
If you don't camp out in front of an Apple store to get the latest and greatest hardware when it's released, whatever you buy will be replaced with something better in less than a year. I got 4 great years out of my iPad 1, and plan to get just as many out of the Air I bought last December.
Other than a lower price, or a larger screen, I can't think of any other way to improve on the Air. Personally, I would buy the Air now if I wanted to upgrade an earlier iPad. It really is a sweet tablet.