Theodore Ts'o has proposed another File System in the future, which is EXT4 (aka ext3dev in kernel) in the LKML (Linux Kernel Mailing List) thread. There has been an article about this topic in LWN (Ext3 for large filesystems and Time for ext4?). I'm not sure whether this proposal will be accepted or not, but extX surely needs to be upgraded due to some limitations that they had which is not adequate for current work. For example, they had 8 TB maximum size of the underlying device it can handle, which is considered too small for big companies. Usually, they had larger device than that. This limitations was caused of the underlying 32 bit system that is used in the ext3 filesystem, so the new file system is being proposed to use 48 bit (i wonder why didn't they use 64 or 128 bit instead of just adding 16 bit from the previous 32 bit? It can be used for the next five or ten years without having to develop or update the structure again).
For those who wants to upgrade, don't worry, since the ext4 code base will continue to mount older ext3 filesystems, as this is necessary to ensure a future smooth upgrade path from ext3 to ext4 users. The development of this new file system will approximately take 6 - 18 months and the first release will be very EXPERIMENTAL. Only those who knows what are they doing should try to use it, as it may break your system (even though the code base was derived from the old ext3 file system). During the development phase, when the developer has agreed that the file system is ready for his new debut, there will be a patch in the kernel itself to register ext3dev as new ext4 file system.
For now on, i'm quite happy with ex3 which is being used in my laptop and reiserfs in my desktop. Actually i prefer Reiserfs since it's faster on my system, but since i installed Slackware on my laptop first, i didn't take the reiserfs as the default filesystem and i'm not changing it now, as it has been fully configured for my need and i'm satisfied with that. Converting the filesystem from ext3 to reiserfs will need extra time and efforts (plus installing other software that i need and testing it).