Interestingly in the last week Yamaha has revealed a crossplane twin cylinder engine, which is very likely to be around the 650cc mark and putting out somewhere in the region of 70bhp.
http://www.motorcycledaily.com/2013/...anese-meeting/
Could it be that the XT660Z could evolve into a twin cylinder machine, to compliment the XT1200Z, with a three cylinder MT-09 engined Tenere sitting in the middle...