1. ensure that there is still an opportunity for something
I'll leave the door open in the hope they'll reconsider
The idiom to 'leave the door open' means to make sure that an opportunity is still available at a later time. To 'leave the door open' also means to allow the possibility for future change or a possible reconsideration of something that seemed like it was already 'a done deal'. You could use this in either social or professional contexts, but it is more likely to be heard in a professional context.