The Hi-fi module peeks my interest the most, but honestly, the "module" thing is BS. In less than 365 days from this point, everyone is going to be talking about LG's flagship of 2017.
Assuming LG is going to keep the ball rolling on this "modular" design, the 1st thing LG is going to do for the G6, is improve on the criticism the current modular design for the G5. They're likely going to make it less clunky, more streamlined. If they change anything about what G5 modules need to work, then compatibility on future models is broken. So alot of what people may not like about the G5 will have to carry onto the G6 to keep compatibility with the modules; because if they don't, and they change dimensions, or functions then all G5 modules are instantly obsolete, locked to the G5's time-frame at the cost of improving their take on modular.
So why spend $70-$200 on a fancy module for the G5, if I'm just going to sell it with my G5 when I'm ready to upgrade. If the module
s can't transcend the G5's model, then it defeats the purpose of "module" that Project Ara started.
But personally, I think "module" wasn't the initial design going into the G5. They were trying to slim down the G5 and go metal, and the sliding battery was their solution. The bottom cap to the battery, which houses the speaker, mic, and possibly 3.5mm port would need the hardware components to work making the actual hardware of the phone two parts. Because the bottom part contained components and was needed to make the phone work, someone on the design team probably had the idea that if the bottom thats still a crucial component to the phone can be detacted, why not make accessories with different functions. I really think they were trying to figure out how to keep the removable battery, and "modules" were just a happy accident because a true modular phone would have several detachable pieces.
