There are loads of planets not on the game that could be amazing especially for a storyline like:

 

Dantooine (has a Jedi academy that could be restored and explored while helping people living in the planet)

Ryloth ( learn more about the Twi'lek species and possibly convince the planet to join the republic or empire)

Kashyyyk( Wookiees would have been good allies against the eternal empire)

Mustafar

Bespin (Could be used by smugglers or bounty hunters

Jedha ( used as a storyline about exploring the history of the force)

Telos (show how the planet has improved since being bombarded by Darth Malak and as a light side user help fix the planet or aspects of it

Onderon

Sullust

Geonosis

Naboo

Ithor (learn history of ithorians and could link in with Telos by getting ithorian aid in fixing Telos)

 

What planets would you like to see join the game in the future?

I'd love to see some old legends planets from Tales or KOTOR... Onderon and Dxun would be great additions in my opinion. Dantooine as well, I wouldn't mind if they created something new however.