This is not possible in near future though both countries want an open border concept. The main hindrance to China is the vast tibetan plateau. Nepal has nothing to gain with a open border concept through Tibet other than for some religious sanctimonies. Also, China still don't recognize Tibet autonomous region under the same visa policy...like for e.g a person issued a china-visa must obtain another visa to enter tibet autonomous region. Similarly, visa issued to Tibet from Nepal is not valid to visit other parts of China. As for economic migration and open border concept, 80% of Nepal's 24 million population living under poverty will gain a lot from emerging chinese cities. But, for this to happen Nepal must first sort out its internal affairs and stabilize its polity and sovereignty. I'd say 20% of the population will migrate or emigrate to China if the concept is put into action in future.