The following info comes from a Famitsu interview with Nintendo's Kosuke Yabuki, the producer of Arms, as translated by NE...
F: I’m assuming that these prototypes are created to test very basic gameplay concepts. So, what kind of concept did Arms originate as? I would imagine it started as a boxing game where the characters’ arms would stretch out…
KY: No, it was not originally a concept for a boxing game. There are many fighting games, like Street Fighter or Super Smash Bros., or even “3D fighters” like Tekken, where the game is viewed side-on. Since the beginning of fighting games, the side view has been the mainstream, but I’ve always thought, why couldn’t we do it a little differently? However, it can be difficult to gauge depth in games where the camera is behind you. So I was absolutely convinced that it would not be suited for fighting games, which require precise distance control. But the fact that your arms extend, and that it takes a bit of time for them to impact, makes it quite likely your attacks will succeed. My idea was to have the game’s tactics revolve less around whether your attack would hit or not, and more around the timing and delay between your attacks connecting. So in pursuing a new kind of fighting game where the camera is behind you and your arms extend, Arms was born.
F: At this event, there are five characters available, each with three different types of arms. Will there be more characters in the retail version?
KY: There will be more characters, with more arm types. What we have available today is just a portion of what will be available in the full game. You can look forward to more information gradually coming out as the release date approaches.
F: You announced a single-player and online mode, but will there be any other modes available?
KY: There are many more that everyone can look forward to… but I can’t talk about them (laughs). Having only 1v1 matches in the online mode would be too exhausting, so there are other modes being prepared. However, the game is boiling down to 1v1 battles in the end, with them as the main focus and the other modes being on the side.
F: I see. What was the reason you chose to use the ‘thumbs up pose’ out of the numerous control schemes available?
KY: During the Switch’s development, we were wondering how we could hold the Joy-Con to play games. Even though it supports the standard grip, the Joy-Con fits in your palms nicely if you tilt them. I thought, if we were to make a game that involved swinging the Joy-Con around, it should use that kind of grip. So, we decided to combine the prototype that became the basis for Arms with that grip style. And, since the internal sensors are more accurate, you can control it with much lighter movement, so I don’t think it will be that tiring. Since there will probably be some people who aren’t good at the physical controls, we made it possible to play using the buttons as well. They can use the buttons on the Joy-Con, or the Pro Controller. It is also possible to play using just one Joy-Con, so you can share the fun and play with others.
F: Can they play without any problems, using only one Joy-Con?
KY: Yes. But their available actions might be a little limited. So you should use two Joy-Con when you’re looking to get serious… that’s how we designed the game.