A power generator is not required if you can charge quickly at charge points conveniently sited along to your route, and you have plenty of range to get from one to the next with some fun riding in between.
I feel like 12 kWh as in Mission R, Zero S ZF14.2 is just barely not enough. 15 kWh is about right; that gives 100 miles of highway range, 150 miles of lower-speed riding.
Charging from 20% to 80% in 20 minutes is fast enough, IMO. That's 180 highway miles/hour or 270 lower-speed miles/hour, pretty close to Tesla's Model S charge rate (340 miles/hour at lower but at much lower power requirements (30 kW instead of 90-135 kW).
Zero is shipping a 12 kWh bike today, at 450 pounds. Mission plans to ship their R sport bike later this year, 12 kWH at 490 pounds and 15 kWh at 510 pounds.
Those are heavy bikes, no mistake; my GS500F weighed about 440 pounds wet, and my 2012 Zero S is much lighter @ 340 pounds. But weight will come down in time, and there are plenty of bikes that weigh much, much more than the 15 kWh Mission bike.
IMO the big technical issues today are cost and rollout/standardization of the charging network, not battery capacity or charge acceptance rates of the larger packs. Initially quick charging networks will be quite sparse, so that a small bike that can charge very quickly will not be useful because it cannot move from one to the other. Eventually maybe it would be useful to have a small bike with 80 miles of low-speed riding that can be charged in 10 minutes from a QC station (i.e. a battery pack optimized for charge kW/kg instead of kWh/kg), but we're not there yet.