ATtiny85 I2C to serial/UART Bridge
The ATtiny85 are inexpensive and the I2C BUS should be able to handle less than a dozen devices fine.
By only sending a short transmission of data (1 Byte can hold 256 values), it would limit the congestion on the BUS.
Unfortunately the design is restricted to using a Nano, although I agree the Mega would be the better choice, especially for designs sake.The second problem with the design requirements are that the main device must be modifiable by simply adding modules.
As for the MP3 Player, I could send a single byte to the tiny and the tiny would translate that byte into an instruction that it would then execute independently of the Nano (like sending an address in an array)
Thanks for clarifying that, for this particular project that is not really a problem as any communication with the "Tiny-YX5300 module" would not happen any quicker than 15 seconds from last transmission.