So because it would allow shapeshift, does that mean the GUI would be able to hold Bitcoin too?
No it means you could enter a BTC address and pay using the XMR in your wallet via xmr.to or shapeshift. Potentially, I suppose a plug in to hold BTC is not out of the question but that is not the originally envisioned use case.
Can the plugin do https://www.changer.com/ also?
A wallet that handles seamlessly handles spending/converting/hodling BTC and XMR is a great idea, because they are complementary.
Ideally, it needs the i2p thingy too! And integrated MoneroDice gambling, of course! Perhaps even OpenBazaar?
We're headed in that direction. We'll get there.
At the rate good ideas get are getting funded, we'll have a Monero blimp over next year's SuperBowl.