Possible minor spoilers for S9 below.
How about this guy?
Lady Me’s journey through immortality is fraught. Heartbreak, memory loss, rage and blame all hamper her adventure. Ashildr cared, and was broken by her caring, and goes too far in the other direction. We’ve gone with her there, and may yet see her again.
But Sam? Sam is potentially immortal too, having received the same treatment. Maybe, as the Doctor said, its potency was lessened by time and circumstance – but he never believed it, and neither do I. Sam Swift wanders the world still.
And his story? He is unlikely to follow Ashildr’s arc. Being older at the time of conversion, and having faced death already more than once, he has a greater appreciation for the gifts long life can offer. He’s also unburdened by caring, having likely experienced and given up on love long ago. He lives for himself; he lives in the moment. I doubt he’ll be cowed by an infinity of moments. To Sam, every one is fresh and new, and every one is to be cherished.
Still, I hope we’ll see him again. I imagine Rufus Hound feels the same.