As I was playing around the Audio shield, I was thinking for setups where you are using low powered speakers, that it would be really useful if adjacent to the line-out pins (L-G-R) there was a pin that brought out VIN. Yes, you have the VIN pin on the standard 2x14 row of pins, but it would be useful to have it adjacent, so you could use a 4 pin jumper cable to connect to the amps and speakers. Or a 5 pin jumper cable to include standard ground as well as VIN (for Teensy 3.x).

For example, using a pair of these speakers (though with these it would be useful if you could also extend the L/G and R/G output one more row and add a VIN pin):

Or an amplifier like:

This is for things like costumes, where you don't need the loudest sound, and instead you want to cram everything together in one small package, and use a single battery.