The next logical step, in my thinking, is a service to move one account from server to server. If you (like most) have more than one character on a server, it seems only sensible that we'll eventually be able to pay a lump sum to move all characters on an account to a new server home -- or even to pick and choose characters from across servers to end up at one single location.
I'm confident that we'll be able to move plural characters around for a flat fee at some point.
A more progressive idea I had, and one I'm not so confident about, is the idea of moving things like pets and mounts from one character to another. I mentioned in a post a while back that I wish our characters could share pets and mounts so we didn't have to re-grind the difficult ones, but what would make more sense is if you could choose to pull pets and mounts off of one character and put them on another (again citing super-rare items like the Riding Turtle, White Raptor, and Black Qiraji Battle Tank) for a flat fee.
Again, my reasoning is that it's a darn shame that some of the rarest items have all but disappeared into the flimsy subspace of stabled characters. In my three years, I've only seen one Black Qiraji mount, which sat in Shattrath for a single fleeting hour before the owner transferred servers or something.
Other people have lost beloved Riding Turtles by putting them on a character they used to play. And don't get me started on dedicated pet collectors who have to re-grind the Hyacinth Macaw.
That one's just cruel.
The truth of the matter is that we're an impatient population, filled with alts, and we don't like to waste time on things we've already done. Blizzard has already acknowledged us with heirlooms, and I feel confident that they'll continue to create and provide services that give us more freedom to revel in the prizes we've slaved for.
On the Devil's Advocate side of things, yes there would be potential hiccups with such a service, specifically surrounding achievement and arena mounts. My reply would be that achievement and arena mounts should be accepted into the service, but that whichever character they're taken from loses them (while keeping the achievement) and can't regain them without transferring them back. This way, if you wanted a red proto drake on a leveling alt, it could happen, but your main wouldn't have it anymore.
Likewise, I cite a friend who got an arena mount on an alliance Death Knight just before he left to roll a horde Death Knight on our server (before we had faction changes). That mount is unused, but he earned it. I say he should have it.
Or what about characters that earn Invincible or other limited raid super-mounts? If those characters get stabled, what does the player have to show for his efforts? Nothing.