I'm starting to see the benefit of DipTrace over Eagle 
Love the 3D models. Personally I'm an OrCad guy, and like KiCad because it's more like OrCad than Eagle... but pretty much all open source hardware people rally around Eagle so I've had to learn it as well.
It's about time someone proposed a new LiPo charging backpack!
I would say add whatever you want for "playground" parts, as long as they can be completely isolated in case a user doesn't want something hooked up to various I/O. The fuel gauge sounds like a done deal. Pull up resistors, check.
What I would love to see is a low voltage cut off on the input of your buck/boost circuit, perhaps programmable with one resistor. Maybe even bridging a solder short could change it from 3.0V to 2.8V or 2.6V. Not sure how safe people want to be with their batteries.
It would be nice if there was room somewhere under the Core, but above the Lipo backpack for a known certain size Lipo. Obviously you may want to just power it with a large lipo that's obviously not going to play. Obviously you can't change this inner volume much so I guess this is just a research part.
A way to "latch" the battery management circuitry into low power mode (shutdown), would be nice. See my post here about low power ideas: (probably not easy to work this in ... but.. if there was a 5V/3.3V mode to your buck/boost, you could power the Core directly on the 3V3 pin and the Core's on-board 5V LDO wouldn't waste so much power when you put the core to sleep).
You can definitely put me down for at least 1, if not 2 of these 