They would have to change the frame, the brakes, tank capacity, and the engine / gearbox – whole new production line – whole new bike with a very different feel to it. They’ve taken it as far as they can with the TW225, anything else and there are market restrictions for Yamaha, (and already market restrictions on the 225).
The TW200 is a unique bike that fits in a unique “niche” – they know what they’re doing. In the UK we only get the TW125 which fits the learner market. Everyone slates it as underpowered except for city life. The TW200 has plenty of torque in third, and is more than capable as a “trail bike”.
Personally, I think that in the scenario you suggest, the price of the 200 would stay exactly where it is today, and there would be little take up for the more powerful model – unless it was subsequently discontinued and became a collectors item.
Yamaha’s attitude is that if you want a bigger bike, then buy a bigger bike – there are plenty to choose from without them compromising a design that’s been successful for 30 years …….