If anything, The only character I liked was sue. I since then like slime girls. In all there are three characters that I'm certain would kill you with ease. Sue may not be one of them, but the only to get a free pass. She makes a great pet too. Kinda... Lala is good too... I feel shame now... oh well...
This gives me an idea. Sue's, like, semi-solid, right? So you could store stuff in her and take it out when you needed it later. Like a sentient, walking handbag. ...Or you could just use a backpack or something, but still.
...I wanna do a chemical analysis on Sue. Some people are all like "I wanna **** the monster girls!" Nah. I wanna do some comparative physiological studies.
Measure Papi's bone density and study her aerodynamics. Does she have mammalian or avian lungs?
Check out Arachne's respiratory and circulatory systems, see how her spinnerets work.
Oh, I wonder if Tio's horn is enervated? And how it grows? Does it work like a goat's horn?
And... Mermaid whose name I always forget. Gills, able to breathe on land or air. Interesting.
Oh, what about Lala? Oh, there's so much that could be done there. An organism capable of sensation without a direct nervous connection? How would that look on a PET scan? Don't gimme that "it's magic I ain't gotta explain **** " thing.
Mero sounds right. I've seen enough SU around FJ that I know a fair deal about it. I wouldn't be as excited about that just because I don't know enough about rocks to really start digging into their biology.
Great. Now I'm wondering how they work. And I doubt they'll explain much in the canon. I think they mentioned purple... Amethyst? being born from a quarry or something, but I guess they don't really expand on the process. Then there's that Lapis one, and her ability to control water. Fusion, future sight, storing weapons in their gem-core things, body transformation, all kinds of things that I wouldn't even begin to know what to do with... Oh, but it could be fun to do a comparison between the gems and the magical girls in Madoka...
Well, the gems are basically just magic gems, and their bodies are like solid projections, which is why the Gem is always attached to them. But the boy, Steven, is half gem, so that alone leaves questions, let alone fusing with a full human, something normal gems can't do. Compound that with the fact that the other is one of the opposite sex (thinking of all confused follicle stimulation hormones and how the body would try and process all the different sex specific hormones is fascinating, but would be a nightmare to try and diagram) And you get layer upon layer of things to wonder about.
Even anatomy aside, are they stronger, like most fusions, or are they weaker since one component isn't a gem? Do they have new abilities, or did they lose some? So many things to wonder and study if they were real .
A good show, though a bit slow to start of, looking at background details and clues for hints are some of the best parts. Fans have actually managed to make really good theories that proved correct. Higher batting average when it comes to fan theories than most I find. If you plan on giving it a try, I would recommended binge watching it to get the initial grind out of the way, as things start picking up (each episode is only 11 minutes, aside from two parters).
On you last note, It would be interesting to compare them, though I often compare the girls from Madoka with Liches. If you don't know, they are basically Wizards that tore out their soul and placed them in an object, becoming undead. They can travel any distance from them, and if they die, the come back in 1-10 days beside their phylactery. Just to become the weakest possible Lich, you have to put anything short of an Archmage to shame.
As for the gems... They are basically dead if their gem is shattered, though I have a theory on how it might be undone and so long as their gem is intact, they can restore their physical being. Plus they are pretty much universally female, so there is that too. Yeah, I can see the similarities.