Using the datamined type images, I've come up with these predictions for the starter evolutions

Rowlet

• Rowlet will evolve into a Stilt-owl, an extinct animal from the Hawaiian islands.
• Stilt-owls primarily hunted on the ground.
• Rowlet will change from Grass-Flying to Grass-Ground in his final evolution.
• The final evolution might have Levitate as its ability.

Litten

• Litten will evolve into a larger feral cat in its middle stage.
• Feral cats are common animals in Hawaii.
• The final evolution of Litten will remain pure Fire type, but will be much larger with more stripes and bomb-themed design additions
• Litten will likely gain an ability to shed its fur to gain health and deal damage.

Popplio

• Popplio currently has the theme of the Hawaiian monk seal.
• Popplio will evolve into a strongman-esque character. He will grow tusks like a walrus that will form the moustache of a sterotypical strongman.
• The bubbles which he is known for snorting will slowly turn into either dumbbells or boulders.
• Popplio's final evolution, and possibly his mid-evolution, will be Water-Fighting.