I think the best option would be to have voice chat with a mute option, as some people here are saying. It doesn't cost a lot to Nintendo to do that. It's not laziness, they don't want that option on the game.
That makes me wonder if voice chat in the game would be very useful. If you see some games in the ranked online mode, it's about conquer one tiny little area in the map. Basically, you die, you jump to that area immediately and try to conquer it. It's not a very strategic mode. It's very chaotic and fast, nothing similar to other online shooter games like Battlefield, for example.
With that in mind, I'm just saying that maybe they are creating the online modes having in mind that Splatoon doesn't have voice chat, making a fast, chaotic and fun game, instead of a more strategic, "hardcore" experience.
I'm not saying that it wouldn't be nice to have voice chat with a mute option. That's, for me, the optimal solution. I'm just wondering about the game online mode design.