Honestly, the only thing we've even had to reskin to create was Entkin, and I'm not sure they count for the thread. I think all we did was kind of merge Tortle and Warforged, add Powerful Build, change them to Plant, and make them require water and air.
Overall, we tend to prefer not having SciFi races in Fantasy settings. Otherwise, I might include Wookiee.
I think Custom Lineage would be perfect for the kind of "mutant" that Witchers represent, although Custom Lineage is so very bare bones at the moment that I've only ever seen it used for powergaming so far.
For myself, I've begun thinking that Darkvision should just be magic. It's just a kind of magic that makes sense for some cultures to teach their children, like a Language. It's also hard to argue that it's not magical because it lets you see in a cave with no light and everything one temperature. No matter how narrowly you contrive the situation, except for magical darkness, Darkvision always works. That's not really physically possible, so it must be magic.
That said, I also think that all Darkvision should come with a rider such as Daylight Sensitivity. I don't think it's fair that some races are crippled in the dark, while others just don't have a problem. Even with the limitations that Darkvision has, it's just not really shown itself to be enough of a limitation to discourage people from wandering around in the dark. The choice between having Darkvision or normal vision needs to be an actual choice with an actual cost or benefit.