An interesting thing happened as I was researching the 500X. In one comparison, it went against a G650GS. The weight and hp are very similar and as one would expect, the X was the smoother road bike and the GS faired better off pavement.
Rather than wait until my Wee got longer in the tooth, I pulled the trigger on a mint condition, low mileage '03 GS came up on Craigs - for about a third the cost of a new 500XA.. I've been having a blast on this thing. It's not only 40 lbs lighter than my Wee, it carries its weight lower. 19" front, spoke wheels and (single front) brembo brakes that put the Wee's to shame, switch-able abs and heated grips are standard, as are a center stand.
So, even though it's a 650, its the smaller, lighter bike that you think has to come with a smaller engine. Not so. (Of course, with 50 hp vs the DL's 66, the single is not in the same league. It's still lot's of fun though between 4,500 and 6k rpms) And this particular model Beemer happens to be rock solid reliable.
(I still have and love me '11 DL650 and it is my "trip" bike. That's what I like: the bikes have the same displacement, but that's about where the similarity ends.)
This is kind of a hijack, so I apologize. If you're wanting a yet-to-be-built imaginary bike, the 650 GS single is not. In fact, it's going to be discontinued. The point is, it already exists. In fact, it has been around for 20 years! Try one. You might be happily surprised.
'11 white Wee "Big One"
'03 BMW yellow F650GS "Roadster" sold
'08 Audi A3 S line Tiptronic
"When you talk, you are only repeating what you already know. When you listen, you may learn something new." Dalai Lama