Debate on a second Splatoon rep? That can be summed up in one sentence.
Series is incredibly popular, but timing is iffy.
Splatoon 2 came out in late July 2017, and since we already got Inklings, I'm not sure if the development team would feel like there's anyone worthy of being a later or last-minute addition. HOWEVER, it's definitely worth mentioning how much of a monster Splatoon has become in Japan. In exactly six months after its release, Splatoon 2 has sold 2 million copies in Japan alone, becoming one of the only home console games in Japan over the past decade to make it past the 2 million mark. Seriously, this game is a phenomenon over there, and I can't stress that enough. It wouldn't shock me one bit if Sakurai saw this popularity and pushed his team to add a second Splatoon character.