Caitlin Snow/Killer Frost end of S4 thoughts and S5 speculation
Definitely spoilers for all of S4 and some spoilers for S5 released so far.
Starting with thoughts on the line from 4x23. I've seen speculation that Thomas refers to either Thomas Wayne, or Thomas Elliot (Hush), a batfamily connection given talk of batfamily for the S5 crossover. I'm not gonna talk about those here because I don't have a background in the comics, only what I read on wikis, and I have some other possibilities I've been pondering.
Thomas refers to her father. If so why would Killer Frost not call him dad? I don't think Killer Frost would necessarily refer to as 'dad' much like she only referred to Barry as Flash until the end of 4x15 despite Caitlin's connection to him. I know the predominant theory is her dad is Elijah Snow but as seen with Dawn Allen actually being Nora Allen, they do like to mix things up from the comics so maybe he'll be Thomas Elijah Snow or even just Thomas Snow but with Elijah Snows powers or Icicle's powers as that's another suggestion for who her dad may be. As for 'he's been this way all the time'..
1a – if her dad is just her dad, no powers, her father could be cryogenicly frozen. Possibly Cait even helped freeze him somehow? This could be something done in the hope his mentioned ALS will have a cure someday. I'm not sure why it would be hidden from her by her parents unless they had to as part of hiding her powers from her and it raising too many questions.
1b – her father has powers like her and did die of ALS but much like when Caitlin died in 3x18, it allowed the alter ego to take over. The way he's been all this time may be his alter ego.
1c – her memories of him dying of ALS are fake ones or it just looked like ALS but was actually him struggling with his powers. He actually left because of his powers and Caitlin either never knew the real reason or has more repressed memories that can dig up the truth.
Regardless of who Thomas is, Caitlin's 4x22 flashback appeared to show her father knowing she was Frosted out after the accident, warning her not to look in the mirror, so, if he does know about her powers, what other possibilities are there for those?
2a – like mentioned above her father has powers himself and she inherited them.
2b – a more unpleasant possibility is one or both of her parents experimented on her. Not a pleasant thought, though her mother's specialty I thought was mentioned to be biology and cryo related unless I am mistaken. Though honestly, it would have much more of an impact narratively if it's Caitlin dad, who she considers her warm and loving parent compared to her emotionally distant and cold mom, who'd done it to her.
2c – I'm not clear how old Caitlin is meant to be in the flashback but I am curious if she's old enough there that Eobard Thawne would have turned up in the timeline. It could be he somehow gave her these powers as part of plan he has for Caitlin and her dad was simply aware that something had happened to her and her parents tried to hide it.
2d – an extra angsty subset of the above would be if Eobard took the form of her dad in order to carry out whatever plan he had and then faked her dad's death before he went off to play Harrison Wells. I'm not sure that would fit with the latter line though, unless her dad is meant to still be alive somehow, so this is just one I include for thoroughness of possibilities that occurred to me.
2e – this one feels fairly unlikely too, but Savitar did say he had a great plan for her. You'd assume that's ended because he's been erased but he did have the possibility of time travel so it isn't implausible Savitar caused her powers in the past when she was a child and this, her having powers, is leftover from that plan of his that got thwarted when she betrayed him in 3x23.
The way they've been talking about her powers being gone but not fully gone has been bugging me. It was mentioned the dark matter had been taken away, assumption that that was why she couldn't use them initially. There was also I believe mention of how she'd found another cryogenic anomaly in her DNA that is what they're trying to trigger but hadn't managed to yet. Ignoring how they'd even know it was a cryo power, if she had more than one anomaly, why didn't Melting Points powers take that too? If it is different and that's why it remains in her, then what does it do and why is there an assumption it will bring KF back? Lots of questions here really. I was initially a little worried they might use that to kickstart KF powers as the heat sucking vampire kind that the comics had, maybe they still will go there in S5, who knows. But it did lead me to another couple of theories on how she could still have meta DNA despite what Melting Point did separating out one of the cryo powers pieces of DNA.
3a - it could just simply be Melting Point only deals with one power removed at a time and she had multiple, so DeVoe only removed the one that had dark matter related to it (which does lead me to wonder if the remaining one could be inherited and the removed one was Particle Accelerator caused, an additional mutation she had).
3b – the more outlandish idea is that Caitlin had an unborn twin subsumed in utero and is actually a chimera (more than one set of DNA in her body) so Melting Points powers only worked on one set of her DNA to remove the meta genes in that and left her with powers via the other set of DNA. I suppose if this were the case they could go for KF being the personality of her twin who's had to watch Caitlin live her life for years with no input, or only come out on a few rare occasions, and that might explain why so annoyed and not exactly attached to people she is, but that's not a possibility I'm keen to see personally and wouldn’t exactly fit with S3 KF.
There's also the possibility that Melting Points powers, to take away the first cryogenic anomaly from her DNA, weren't the only thing DeVoe did to Caitlin.
4a – did he place a mental block in her mind?
4b – could he have placed fake memories?
4c - ??? Honestly, DeVoe seemed to have figured out how to use the powers he had in unexpected ways, and could also use them in combination, so there could potentially be a big surprise coming if he did something very unexpected rather than simply steal her powers. But I think I am all theoried out here so no speculation on other stuff from me for the time being.