Imagine playing Mario Kart, except to go, instead of holding down the 2 button, you pedal on a Balance Board (like the Bike course on Wii Fit U) or run on the Active Life Mat? The Game Cube Port adapter needs to work for more than just SSBWU.
Some games the balance board works fine, but other games you need to run real fast, and a Balance Board can't handle it for pure calorie burning, because you'll either break the balance board or your ankles when running real fast. but the Active Life Mat was meant for speed.
The Active Life Mat may or may not be handy in Mario Kart where a Balance Board will do, but imagine it in Splatoon, where running in humanoid form requires you to run. The faster you run, the faster the squid runs. You can't do that well naturally on a Balance Board.
This adds extra strategy, if you don't have the speed and endurance of a pro athlete or soldier, you've got to pace yourself, and use speed bursts strategically, unlike most other shooters where you go all out all the time with no consequences.
This will put the sport back in E-Sports, and let a division where people who are athletic beyond their thumbs excel. Also, all my local friends were way too athletic, and no one would pick me. Online anonymity would let the less athletic get some exercise without feeling inadequate, especially in a team game like Splatoon.
Nintendo started the revolution of getting gamers off the couch. Finish the job by making some lower body, combined with WiiMote upper body exercises in a Sports situation. Not just a gym instructor measuring how well you do, but to play games with real live remote human opponents. Games where running speed is an aspect, but not the sole determining factor.
Of course, we won't take away traditional modes, and to encourage exercising, the exergame division is a separate division for rankings and matchup purposes.
I talked to a company who lied my idea but said the Nintendo Third Party system is too bad (in their opinion) for independent developers.