If a new EU treaty has nothing to do with Americans, then Americans have no right to vote on it. If the voting public is in Europe, then they have every right to exercise their voice and consent on treaties that ultimately effect their lives. Referendums like that in Europe are common.
Career lawmakers lose touch with what real life is like. The people who continue to live and work outside of government are deeply involved with society and feel the impact of every law enacted. Furthermore, citizens were the ones to elect the lawmakers in the first place. They shouldn't lose their voice simply because there is now a representative holding an office. With more input on what real people are thinking it can be helpful in creating laws that work better for the country.
It goes without question that any new treaty that is proposed by the EU should always have a referendum. It is important that such organizations are not allowed to act without consent or approval by others when dealing with thing as important as treaties. That is why any new EU treaty should have a referendum.
A referendum's validity is only ensured by maximizing participation rates across the population, but with so many people covered in the EU, and so many possible treaties being considered, the goals meant for a referendum to achieve would be stunted by structural factors. It would be better to maintain the status quo in regards to treaties.
No, I do not think that there needs to be an referendum held on and of the new EU treaties. I think that this will just end up messing up the system a whole lot and that it needs to stay just like it is today, and not change at all.