One thing that would be super cool in GFS2 is the ability to tack on another block device and expand the filesytem. Possibly using stripping, and with more volumes parity such as RAID456 etc. or configurable.
What would be also cool, would to be able to remove a block device as well. (dependant on free space). The data would move off in the background and the block device would be released.
The usage case for this is where we want to provide a very high performance cluster for video on demand. We would like to use RAMSAN with GFS2 so we can use lvs to load balance the cluster. In terms of growing the space, we could shard the data across additional RAMSANs added later. However we would then have to factor in usage patterns in the sharding, and would find ourselves with 'pockets of unused performance' - in the sense that a new episode of 'Lost' would be hammering RAMSAN a, whilst b is doing nothing.
In this case, it would be ideal to be able to expand the GFS2 on to the second (or third of forth) RAMSAN and have the raid spread (raided) across them.