You can't revisit Zion Valley and you can't go back to the Sierra Madre. Is the Big MT the only other place you can revisit, or can you go back to the Divide?

Umm, you can revisit Zion whenever you want, so I have no clue what you are banging on about with that regard. :/ As far as the Divide, yes you can return. DM is the only place that you can't revisit once completed.

The only DLC you can't return to is DM.