How can cryptocurrencies be integrated and promoted in mainstream financial systems?

Could you elaborate on the potential ways in which cryptocurrencies can be seamlessly integrated and actively promoted within the framework of traditional financial systems? What are the key challenges and obstacles that need to be overcome for this integration to occur, and how can regulators, financial institutions, and cryptocurrency enthusiasts work together to facilitate this process? Additionally, what strategies or initiatives have been successful in the past in promoting the adoption of cryptocurrencies in mainstream finance?