Funnily enough USB's inability to work over longer cable lengths works in its favour here. Generally we don't rely on anything working over 3 metres.

Wikipedia says "the maximum power supported is up to 60 W at 20 V, 36 W at 12 V and 10 W at 5 V" [1]. For a typical 3 metre 20 gauge USB cable 10 W power delivery will cause a voltage drop from 5 to 4.6 V. This is within the +0.25/-0.55 specified for USB 3 [2].

Since nobody can rely on the 5 V from USB to be exactly the charging voltage they need, there will probably be switchmode regulators in-line anyway. Device manufacturers will just have to spec these up a bit to accept higher input voltage if they want more than 10 W.

[1] http://en.wikipedia.org/wiki/USB_Power_Delivery_Specificatio... [2] http://en.wikipedia.org/wiki/USB