I found that this build has dominated me a few games of dominion. Item build makes Riven ridiculously powerful on the Crystal Scar by the mid game. Now you want to get warmogs armour as quickly as you can because you need time to stack it. Next you should get either the frozen mallet or Atma's. Either way works well.
If you get the mallet first you will be doing very little damage, but when you get Atma's you can catch the other team off guard by dishing out massive damage.
This bulid can fill a number of roles on CS because of your durability you can easily hold the windmill by yourself in a 2 v 1 situation. But I've found this build works best when you can jump on opponents trying to take objectives.
In terms of map position you want to be where the action is because you can dive right into the middle of a group of bad guys rip out some damage and still have hp to spare to take a lazer to the face or kill a full hp teemo.
When you combined this with Riven's natural escape abilities you become very hard to kill and can easily get kill streaks and other good stuff.
The frozen mallet is great for champs who try to backdoor you because once you catch them there is no escape.
I have never had time to build the FoN in a dominion game but I figure that it would complete the build nicely.
Quite an easy bulid that allows you to fully enjoy using Riven's abilities to the max without the panic of having to spam you abilities in the one second you have before you die which I see happen to most champs on dominion.

This has been my first build so please tell me what you think.