Really the main thing the USA could do in terms of sanctions would be to tighten the grip on Iranian banks. Allow no primary dealings with Iranian banks with US banks/funds/markets. In addition to this do not allow secondary banking with them. In other words if a bank in another country dealt with Iran, or money was given to said bank from Iran, that bank does not do business in USA. Has been done in the past.
Generally speaking, "You realize other nations follow their own foreign policy and it doesn't always align with ours right?", that is my point. Whether agreements are signed and/or followed or not, other countries are going to largely do what they want to do anyway. That is what happened with the NK and Iraq pacts. That is why largely for me, this is just a non issue. Obama sells it as a triumph, Republicans say it is a disaster, for me, just do not think it matters much. Israel, SA, UAE, etc think Iran is to much of an issue they will do something outside the agreement or even unilaterally. Same for the USA. I think Russia and China generally support Iran's spreading of chaos in the ME and will give Iran more latitude.