If we ever get around to it, for the next content chapter or whenever you add more companions to the game, I like to see a few CP slots focused on the companions independent from the rest of the tree.
Craft
???: 50 points, 10 points per segment, passive. Increases your chances to find companion items by 2% per stage, up to 10%.
Warfare
The More the Merrier: 100 points, 0 to 100, slotted star. Increases the number of companions you can have active at any time by 1, but reduces companion's experience and rapport gains by 25% and only when you're not in a group.
Full House: requires "The More the Merrier" 100 points, 0 to 100, slotted star. Further increases the number of companions you can have active at any time by 1, but reduces companion's experience and rapport gains by 33% and only when you're not in a group.
The trade-off is you're losing out on potential output by giving up 1-2 star slots to have extra companions following you around. Furthermore having additional companions causes a drain on how much experience and rapport they can gain (does not affect rapport loss) so you're encouraged to leveling one at a time. Lastly joining a party or group disables them reverting it down back to a single companion without the experience and rapport penalty.
Fitness
Companion Expertise: 40 points, 8 points per segment, slotted star. Increases the potency of your companion's item traits by 4% per stage, up to 20%.
Aggressive, Bolstered, and Soothing go from 20% to 24%.
Vigorous, Quickened, and Augmented go from 31% to 37%.
Shattering goes from 15,600 to 18,700 penetration.
Prolific goes from 156% to 187% Ultimate generation.