Those who were with Netaji during the said crash have said that he was injured, but not dead. So there may be something to the Russia - Siberia theory.
India's first Prime Minister Nehru and his Congress party were not exactly friendly with Netaji. And the conspiracy theory folks think that Netaji may have been left to die in Russia after India's independence due to pressure from Indian govt led by Nehru and Co. Since Nehru dynasty was very close to then USSR
, some think that Russians have destroyed all evidence decades ago. So nothing related to Netaji was found in USSR
documents that were unclassified after the end of cold war.
Other theory says that Netaji returned to India in disguise, but was disgusted with how govt is run by Nehru and co and died as a swami somewhere in Himalayas.
Netaji controversy raises its head periodically with sightings etc - I guess kinda India's Elvis