Re: Sorry LG, the idea is doomed.
Well I'd go for it.
Better grip and extra battery, together with dedicated camera button - why not? Agree that it is a niche product, but that'd be the whole point of this kind of modularity - if I'm going on Holidays, I'd take this attachment. Off to the office, I won't bother.
Whether this works depends on who LG partner with (or better yet, make it open source and see what comes your way), but high quality DACs are wanted by the audiophile market, and you've got potentially many offerings for those who want something a bit different within their own niche.
How about a FLIR attachment? night vision, an extender offering a USB hub? Much higher quality microphones, or a conference speaker set up. Laser measuring devices, LED projection equipment. If this was done properly then it could really be a full differentiator and actually really rather useful. There are plenty of things that could be used with the processing power of the phone which would add too much bulk by themselves (or their permanent inclusion would just provide too much of a power drain) but incorporated into an add-on like this and designed for temporary use could work rather well.
We'll see. If Apple had come up with this then everyone would be praising their ingenious foresight. As it's LG, well, someone has to continue the innovation to let Apple "change the world" by including it in their products in 5 years time once the bugs are ironed out. and then sue you in 10 for "copying."