So can this port be used as UART? When yes my friend had developed OPL codec PCB BOARD based on ARM MCU... You just send data and board plays it. No 68000 CPU time...
So we can have OPL soundcard (stereo, lot of channels) for Genny than...

Here it is:

Seeing as the Sega Genesis's use of the expansion port for the model 1 and model 2 is solely for things like the Sega CD and the unreleased floppy disk drive. The Genesis's interface through the expansion port are extremely hard to deal with because while you are able to write directly to memory and even use a secondary (technically tertiary with the Sega CD) processor, you are having to deal with all the issues of the Motorola 68000 processor's predisposed uses for the Sega Genesis. It, in the end, is a decent platform to program for but I would not recommend it. The programming community is decently active, but you may have trouble breaking into it. Try checking out www.gendev.spritesmind.net for starters.

Right now, there is no real need to access this port beyond what the Sega CD gives us. It allows for booting custom code, cartridge ripping (to a degree), and easy-to-distribute-code for testing on real hardware (easier to burn a CD than buy and flash a cartridge). Given, the Sega CD is much harder to program for than the Genesis. I hope you can explore these endeavors it's a cool thing, but most casual homebrew/vintage game hardware hacker people don't touch the Sega consoles.

Hmm what kind of DIY devices can be developed for this port (in future):