What can I use my extra dragon souls for?

There is currently nothing you can do with extra souls. It would be nice of a mod or DLC came out that allowed you to turbocharge a shout at the expense of a soul or something.

There is currently a mod that allows you to trade dragon souls for perk points here if you're playing on the PC. Other than that, I can't think of anything.

On completing the Dragonborn DLC quest line, you have the ability to use dragon souls for clearing perk trees. This gives you back the perks spent in the tree while keeping your skill level in that tree the same. If you've got the souls, you can do this as often and with as many skills as you like.