Wow, lots of cool suggestions!
I thought the clear favorite would be the Rotax 654 as used in BMW F650 and G650 series bikes. I have got to admit, not exactly thinking out of the box, but it ticks most of the boxes. Pretty sure I even read on here about a guy in the middle east building some rally bikes out of KTM 660 frames with the BMW engine?
But I really like the idea of a twin. I only thought about the V-Strom and was sad that it would clearly not fit. Never even heard about the Versys before you guys mentioned it, but now that I have done some reading this thing seems perfect! Compact inline twin, 650cc, 6 speed, FI, 55-65mpg, 63HP/45ft-lbs, readily available for less than $1k even here in Europe. Now I just have to figure out if this thing will fit, especially since it's left hand output.
DR Big engine is a bit too old school for me, for similar power I could just put a big bore kit on my DR650 (might still do that one day). Yamaha 660 is also a good choice, but doesn't offer enough advantages over the Rotax to justify messing with left hand output. And as much as I like the new 690 LC4 (only ridden the carbed version so far), I would rather wait for KTM to iron out the bugs in the EFI, change to a wide ratio 6-speed, and finally sell it in the 690 Adventure version we have all been waiting for!