It would be nice if there was a "--leave" parameter to the kickstart part and logvol commands that went with --grow and specified the about of space to leave free. This is especially useful with LVM to provide a pool of free space for future growth.
Adding FutureFeature keyword to RFE's.
I'm willing to consider patches to do this, but at this point I don't see that I've got the free time to work on it myself.