So, what is Ground-type's role, really?
The Legendaries and Their Role:
As said before, Groudon's association with Ground is because it represents earth's tectonic plates and lithosphere in general.So, what is Bug-type's role, really?
As said before, Landorus represents fertility, and its association with Ground represents tilling of the earth for cultivation.
Zygarde's symbolism will be discussed better in Dragon-type, but its association with Ground represents Earth's association with biosphere, evolutionary progress and circle of life.
The Trainers and Their Roles:
"The Self-Proclaimed Strongest Trainer" Giovanni is a Yakuza boss obsessed with power. His rule over Team Rocket represents his belief in the power of the many, while his persona represents his belief in power of the self.
Elite Four member of Sinnoh, her association with Ground-type seems to be based on power of experience. In the animé, her battle style is focused on observing and utilizing the field.
"The Underground Boss", Clay is a Japanese immigrant who got rich in mining industry and went native to America. His persona represents power of perseverance.
Most all Ground-type legendaries represent cycle of earth's biological and terrestrial change. There is no Legendary that represents an individual part of Earth's ecosystem, just different spheres of interaction.
Most Ground-type trainers also represent power, but they are all full of surprises, either the power they have attained seems in conflict with their weaker visual presence, or they have something to hide, even if they eventually expose that. Most Ground-type trainers are also convinced of obtaining power, either for good, bad or selfishness, as it is the only thing that allows them mobility.
It is hard to say if we will see more micro level Ground-type trainers and legendaries, but Ground-type is relatively versatile to provide something a bit more down to earth.