Apple could enable users to store all of their media on a central server by getting iTunes clients on a local network to cooperate and coordinate their reading and writing to the central library and files over the network, without requiring significant server-based changes or sophisticated software. However that would not address the desire of users to access their media from remote locations, and a simple device like the TImeCapsule would be incapable of doing the transcoding required to stream high-bitrate video across relatively slow networks.
The most important change Apple could make with a theoretical home server product, would be to address the data integrity problem others have already solved. Enterprise customers routinely use multiple redundant drives to ensure data is never subject to a single hard drive failure, the sort of failure that TimeCapsule users are, for now, highly vulnerable to. Typical solutions to this problem include mirroring data across multiple independent machines, or mirroring data across multiple drives in a single purpose built machine such as Apples own discontinued Xserve RAID device, or Data Robotics inc. Drobo, both of which use sophisticated logic to replicate data across multiple drives transparently to the user. When a drive fails, the drive array can recover without losing important data.
Apple however has a trick up its sleeve here, they have been working with Sun microsystems on porting the ZFS filesystem to Mac OS X for some time now, and have made it known that ZFS will be an option in at least the server version of Mac OS X 10.6 Snow Leopard. So might Apple be planning to use ZFS in some fashion to provide home users with safe, redundant storage? Such a move would make sense, it would remove the need for expensive dedicated RAID hardware while retaining all of RAIDs advantages.
Whatever they end up doing, there are clearly needs here that aren’t being met by Apple products alone, however Apples recent discontinuation of the Xserve RAID, and their cooperation with HP on getting their HP Home Media Server to work with Time Machine, may be an indication that Apple doesn’t want to be in this specific market, and may leave it to a third party to solve.