Joining late to the Joy-Con sizing discussion: While I have no inkling of potential new Joy-Con features, I doubt that its general dimensions would be changed much. Unlike Sony and MS, Nintendo is targeting a less homogenous audience. The design of Switch is therefore required to accommodate a wide range of anthropometrics. Look no further than the hand and grip sizes:
Global adults, 5th to 95th percentile (
source)
US children, 5th to 95th percentile (
source)
While IMHO Nintendo isn't likely to alter the Joy-Con's dimensions beyond a few millimeters here and there, I hope that they would officially license their controller merchandising to numerous 3rd parties, resulting in more varied products in different sizes and shapes. AFAIK, today only Hori, PowerA, and PDP are licensed, and all of them too cheap to put HD Rumble in their controllers.
I also don't believe that the Joy-Con rail standard will be abandoned anytime soon. As a platform owner you want to a) lock in your users through proprietary technologies, and b) facilitate network externality by maximizing the install base of your technologies. Look how Apple fought EU tooth and nail to keep their Lightning port. Even though Apple eventually lost, it bought them a few extra years of locking users in the proprietary format. For that reason I suspect that Nintendo will ride the Joy-Con rail for at least another gen.
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