I agree, new worlds would be better rather than going back to, say, Dantooine for the third time and Mustafar and Hoth really would leave you with nothing to do.
If Corusanunt is included in K3 then I don't think Correlia or Nar-Shaddar would be there as well, fact is that they are all city worlds-Corrleia less so-so they would end up being to similar in appearance and probably in possible missions-at the least it would stretch the abilities of the development teams to the point where they would no doubt consider dropping at least one of them. Having said that Corrleia has something like five inhabited planets-and at least two of them are not populated by humans by native species-as well as Centrepoint station. So they could include Correlia along with Corusant, as long as most of the action on the former takes place elsewhere in the system.
Alderraan would be a good alternative to Dantooine, the planets are, I think, meant to be similar in apperance although Alderraan is the more heavily populated world.
i agree with both of you there needs to be more to the planet than just one station or just the main city. if they did include corusaunt then they should leave out nar-shaada and corelia or possibly go to crorelia for center point station or something and the sith worlds need more interaction within the system and not be so empty koriban being the exception since it is just a baren waste land/burial ground