As much as I love Canderous, I think he should appear as faction leader of the Mandalorians rather than a permenant party member, much like Carth appeared for the Republic in K2. Though I'm in two minds about HK, I think T3 should be recruitable, as it was there for the majority of Revan and the Exile's original quests, so it would make sense if it met up with the K3 protagonist at some point.
I'm not sure I'd want any non-T3 NPCs to be the new party members, unless they're cameo recruits. I'd actually prefer to see a few different cameo/temp recruits across the different level in addition to your core party. If K3 is about the battle in the Unknown Regions, it could be assumed that the battle will break out into the Outer Rim, at which point the Jedi (be it the Exile's Order or Bastila/Juhani), the Mandalorians and the Republic, among others, would be present on some of the playable planets, and may be willing to work with you for as long as you remain there in order to further their goals.
Though toggle/optional characters are good in theory, I'm one of those people who would have preferred to have both Disciple and Handmaiden, as though I didn't hate Disciple (that much), I thought Handmaiden was more important to the story. If there's going to be two characters I have to choose between, I want to be able to make that choice as I play.