I'm unaware of a particular compelling argument for another scalar. Scalars are easier for theorists to use when trying out a new theory, so they abound in theo

I'm unaware of a particular compelling argument for another scalar. Scalars are easier for theorists to use when trying out a new theory, so they abound in theoretical physics. At some level, it is something of a surprise to have found that there is just one particle responsible for the Higgs mechanism, and it is indeed spin-0, as Nature hasn't given us one of those before. Prior to the discovery of the Higgs, perhaps the majority of physicists were betting on something Higgs-like, but not quite the pure vanilla Standard Model Higgs. I would not bet any money on it, but if one were looking for another scalar, one might find it in whatever mechanism underlies dark energy. Many theoretical models for dark energy use scalars, again, because they are easier...