the iPod form factor continually changes, so it's kind of hard to make an "iPod-shaped hole"
They could take a tip from those 10-in-1 memory card readers. Somehow, with only two slots, they can fit all the different types of memory card. What I'm getting at is adapters, like microSD inside SD. Build "the hole" for the largest possible (iPod Classic?) and build in adapters for all the others. I can picture a built in solution, with retractable sections depending on the size, but it's hard to describe.
current car stereos where you can just plug in USB,
There are tons, unless I'm misreading this. Check out Crutchfield and you'll see everyone offers this now.
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FireFox31
110gig MKIIa (30+80), Eutronix lights, 32 meg stacked RAM, Filener orange gel lens,
Greenlights Lit Buttons green set