Just infodump about Sam to me <3 Literally anything
I love Sam. I just think he's so neat. I think I'll talk about some of his relationships with canon characters.
I like how when Sam starts crushing on the Doctor, it's less of a schoolboy crush and more like "I wanna put this guy under a microscope", and it's such a surprise to him when the crush starts developing into romantic love. And even Sam doesn't completely understand how and when it happened (spoilers it sparked at the end of Gridlock when the Doctor is telling Martha, Maisie and Sam about Gallifrey).
Sam and the Doctor are such an enigma as a couple. The Doctor is pretty much the opposite of the type of person Sam has gone for in the past. Maisie points out several times how funny she finds it cuz she didn't think the Doctor would be Sam's type at all. But Sam can't help it. He just loves that look the Doctor gets when he's excitingly explaining something and the overexaggerated gestures he makes.
I love the eventual friendship he develops with Donna. One of their favourite pastimes is lowkey bullying the Doctor (in a loving way, you know). I have this idea for Silence in the Library where when the Doctor tries to transport the two of them back to the TARDIS, their consciousness get glitched together. Causing this deep look into each other's psyches. They start to understand each other's loves, dreams, fears, and anxieties on a metaphysical level (is that the right word? *shrugs*). It would also be how Donna unwittingly finds out about John Smith, and how Sam is still haunted by what happened with him. Least to say, Donna is totally gutted when she finds out.
It makes Donna's ending hurt even more. Like can you imagine having a friend that understands you on so many possible levels. Someone who understands what it's like travelling with the Doctor and all the baggage that can come with it, and then that friend is forced to forget.
And then there's the Master! The current version of the story outline has Sam leaving the TARDIS after the events of Family of Blood. The Master finds out about Sam and his connections to the Doctor, and Sam is forced into a situation where he works for "Harold Saxon". So for a full year before the events of last of the time lords Sam basically has no choice but to endure working for the Master so his family isn't killed or worse.
What would be the best way to describe their relationship? The Master would definitely have a curious obsession with Sam because he basically hears how the Doctor talks about Sam during Utopia and wonders, "who is the person that has the Doctor so 'enamoured' and how can I use this to my advantage?"
Sam, on the other hand, has never felt true hatred for anything until he met the Master. Sure, he's felt fear toward something like Daleks or Cybermen, he's been angry/felt disdain at horrible people, and he even thought he hated the Doctor at some point, but it was nothing compared to the pure vitriol he had against the Master. And the worst part is: That's exactly how the Master wants Sam to feel about him.
Like you know that one bit in Undertale where you have the option to kill Flowey and if you do it, he just laughs at you. Taking away any satisfaction you could have gotten from killing him? Sam has a similar option. He has the opportunity to kill the Master at the end of season 3. He doesn't go through with it, but if he did-
-That would be exactly what the Master wants.