I spent some more time thinking about my ideas and changing and polishing some more details:

The plan for Lyla: Have her gagged with a ball gag at the start. Start out with some rape, have her tied with spread legs and arms on a table. Then go on to some needle torture, put some under her finger nails and then focus on her tits and clit. Now you finally remove her gag and get to work on her teeth, take out all her teeth! If somebody wants to fuck a girl in a teethless mouth, now is the time. Then gouge out an eye, stick it into one of the needles in her tits, leave the other eye for now. Maybe you could rip out her tongue, just have to make sure the blood loss isn't killing her, if you do stick that onto one of those needles too. Then it's time to amputate her arms and legs, maybe with a chainsaw to make it clean and fast. Either cauterize the wounds or give her some bandages, so she doesn't bleed out that fast. Now it's finally time to take out her last eye, stick that on another needle in her tits. Then put her upright on a huge metal dildo, put the gag back into her mouth so she is quiet, and just leave her there as a room decoration. When the next girl is brought into the room, she is still alive, to increase the horror in that girl. Eventually she dies from her injuries and is left as a dead room decoration. While it's probably not easy, i think this should be possible to do.

The plan for Latisha isn't that concrete. I kinda want her sister and maybe even her cousin involved. Since souro claimed Iris, she could just have to watch it and maybe get a taste with some brandings if she protests, until she gets taken away for her own torture. Lilly should do more than just watch it, maybe you start with both of them, but for some reason the torturer focusses on Latisha, maybe she gets cheeky, she looks like a girl who could throw some insults at her torturer, and so she gets tortured more than her sister. I want to see good use on some hot irons all over her body, and i want to see those used in her ass and pussy, and also to burn out both of her eyes, maybe just like with Lyla start with one eye somewhere in the middle and save the last one until close to the end. Instead of hot irons, you could also use a welding torch for some of it, cigarettes could do some work too, and maybe even some simple candles. Candles or hot coals in the right position under her body could also make things more uncomfortable over time, maybe you could at some point just tie up her sister in a position over candles or hot coals and then focus the more active torture on Latisha, or maybe shift around between those two for a bit. I don't think her tits are big enough for some real tit torture, but that could work with her sister. You could tie Latisha into an upside down position and force some boiling water down her pussy - that should be lethal, but wouldn't kill her just yet. Not sure what more to do, boiling alive might be too hard, burning at the stake is almost certainly not possible the more i think about it. I hope everything else should be possible.

Celine should end up with lots of broken bones and her broken arms and legs twisted in some unnatural positions, ideally on the wheel, but just make whatever comes close to that and is possible for you to do.
Gwen would be my favorite to torture on a wooden horse. That should probably have a spiked dildo in the middle, to make it even more uncomfortable to sit on it. There are all sorts of things you could do to her while she's sitting there, i'll leave that to you. I'm not sure how to make that lethal, the only idea i have is to heat up the dildo inside of her and cook her from the inside, but something similar was requested a bunch of times already. She is also a girl i would like to see gagged at some point.
That would make Nina the one to get crucified, looks good to me. There are a bunch of things you can do to her while she is hanging on the cross, i'm sure you have some good ideas.
All of that should be possible.

Going over some suggestions others have made:
I like souros suggestions, seems fine to me.
I like most of LadyCaths ideas (i'm not into pissing, but waterboarding in general seems fine). I love Estas ideas, since both posters have ideas for Devon and her sisters, maybe it's possible to combine them and start with waterboarding, then move on to the cooking event - just not sure how much of that is actually possible to do. I especially love the suggestion for Jinn, just don't know how much of that is doable, but it combines two of my most fucked up fantasies, one with ants and one with pouring molten metal into a girl. I have seen Krogue do some good looking work with ants on lolicit, up to that point i didn't think i would ever see that, so it might be possible. The aluminum cast might be impossible to do, but just pouring molten metal into her pussy should work, at least i hope so.
I like the ideas from Killerfox, especially the one for Lorin, the idea for Rica sounds hot too. And i might like a hanging scene like that, at least i'll like it more than a simple hanging.
Magali's idea to play with some hot sauce sounds fun to me. For the eye fucking you need to get rid of the eyes first, and that's always a great idea. The board game seems like a nice idea, but i think it would take way too long, that alone looks like it would take a few hundred pictures.