Fluidium has a really interesting premise that allows the creators to play a whole lot with the concept of gender. Basically, folks are born with two separate male and a female bodies, and until they are 21 are free to "switch" back and forth between them. (It's unclear to the point I read exactly how this happens.) At 21 they must make "the choice," and stay in one of those bodies forever; the other disintegrates into nothing. The protagonist, who just had their 20th birthday, feels at home in both bodies and dreads making the choice.
The creators slowly build out the world answering interesting questions like pronouns, family dynamics, the societal construction of "a choice," and the process of finding out what body one is most comfortable in. I found the world fascinating, but it was difficult for me to stay engaged with the drama of the teenage characters. That isn't necessarily a bad thing, I just think this webcomic is made for a younger audience in mind.
That being said, I am so glad this webcomic exists, and I can imagine so many people both feel seen and represented by its message, and it's also an important message for those of us who are cisgender; we never have to think about making a choice, and it begs the question what our relationship to gender would look like if we did.