Like Dragonrage said, juggernauts are very hard to fight as a melee champion. They might be a lot less mobile then a Riven, or any other full melee champion, but their lack of mobility does not matter much when both champions have to stand near each other to do damage.
If you know you are going up against a juggernaut and can still pick something else, then do so. Most of the time I pick Gnar against the juggernauts because as Gnar I can kite easily and can deal max health damage. Once my rage bar gets full I can even turn in a juggernaut myself! Fiora works as well because of her max health true damage and her ability to completely negate all damage from a Darius or Garen ult.
If you don't know who your opponent is your best bet is to just hang back and freeze the lane near your turret. By doing this you make sure the juggernaut has to over extend to farm which allows for easier ganks. But if you have enough skill as Riven and you can win trades like you said you can then freezing and when possible harassing the enemy may work very well.
Just make sure a Garen doesn't silence you, a Darius doesn't pull you back in and Illaoi does not grab your spirit. These skills are by far the most dangerous as it allows them to deal heavy damage to you.
And by all means avoid giving the juggernauts an early lead. If they do have a lead even a gank from your jungler is not going to help much and will most likely result in two deaths for your team. Sometimes I tell my team not to help out my lane. This way they can win the other lanes while I make sure the juggernaut doesn't push too fast.
I can't give you any information about Nasus. Preseason is not being kind to him at all.