You need kids! Well, at least kids sizes.
I understand the desire for adults to have a shirt that fits, but I think that falls squarely in the bucket of "nice to have". As a parent of a child that doesn't fit any of the sizing charts, being able to predict what size of what brand will fit my kid would be a "must have"! Kids also tend to go through clothes a lot faster than adults, making the potential savings of getting a good fit far greater.