I wouldn't say that Spring is constrained by the Java language. There are other JVM languages that work equally well with Spring (Groovy, for instance--Grails is based on Spring).
Even so, just because the language evolution seems to have come to a crawl, that doesn't mean that there's not work for Spring to do. New stuff comes up all of the time where Spring can jump in and help developers out. A few Spring projects come immediately to mind: Spring Social, Spring Android, and Spring Data. These projects came about to address needs in areas that really aren't language specific.
As for "what's next?" I'm not sure I can say. I'm not keeping secrets...I've just been mostly focused on Spring Social (and to some degree Spring Android and Spring Mobile) for the past year that I've probably overlooked the "wow" kind of next thing you seem to be asking for. For me, the "what's next" question has been Spring Social 1.0.0.RC2 for the past few weeks...as of about an hour ago, that has become "what's now" after I pushed out the release.
If you really want to know what's going on in the Spring world, I highly recommend that you attend SpringOne/2GX. You'll usually leave with a fairly good idea of what's being worked on and what to expect over the next year.
