I completely 100% agree on this, as I always wanted this to happen ever since I first started playing SWTOR a few years ago. It's definitely something I want the most out of anything in the game.
It will be very cool have a legacy companion, and the special thing that I'm talking about, is to have it join you in a conversation in flashpoints as if your teaming up with a player, like in this old SWTOR video.
Outstanding suggestion, I love the Galactic Leader one the most because I always wanted to use a 2nd companion in a Solo Flashpoint. The solo droid is getting super old now.
Companion standard attacks really need to have less delay, just like how are standard attacks have hardly any delay. Let melee companions have the standard attack of the Sith Warrior and Jedi Knight and let range companions have the standard attack of the Bounty Hunter and Imperial Agent.
Like what the title says, we really should have the option to replace the droid with a 2nd companion, it shouldn't be to hard for them to do it anyways. Not only that but it would make Story Mode Flashpoints even more fun that way.