First Unlimited Text, Then Unlimited Voice, What's charged for when Data goes Unlimited?

So far, there has been a trend I've been noticing in Mobile plan offerings.  First companies charged for Both Minutes and Text Messaging. Overtime Text Messaging because something that they eventually offered in a Free Unlimited Bucket.

Then, as phones really became pocket carried Computers, Text Messages and Voice minute buckets both became part of your Base Rate Package, with the charging item not being Data, but that lead me to wonder about something.

What's left for Mobile Carrier's to charge for? With Both SMS and Voice Minutes pretty much only having the cost of System Access being charged, and phones having the ability to access more and more Wifi based services. What do you, the users, think will be the next thing Carriers can charge for, Please leave your comments.