I'm fleshing out a magic system, but I'm indecisive about reasonable rules for shape-shifting.
Background: In this world, magic requires years of training and/or studying to even do weaker spells (healing minor cuts/burns, making a small flame, etc.). Most major spells (healing severe injuries, making a big flame, etc.) require multiple people to be done. (There are objects that can enhance magic/cast minor spells for people.) Magic done improperly can be volatile and even harm the user.
Because of the learning curve, magic has been confined to certain the clergy. However, the printing press has allowed an unprecedented number of books to be circulated (including ones on magic), changing this dynamic. However, since the printing press, there have been increasing numbers of monsters* possessing people.
*The monsters are loosely based off of evil eye. They usually appear as everyday objects though cling to people a bit like the Spiderman symbiote. They sense people's envy and insecurities and use that to coax people into being possessed. The possessed are often transformed into their "ideal" self. The possessed often need blood to maintain their transformation. (Possession tends to mess with the mind, affecting judgement.)
1) Should there be benign cases of possession by these monsters (I.e. some people not actively seeking out blood, feeling content, etc.), or does that subvert the rules in an ineffective way? (One of the themes is about oppression from societal expectations, but I don't want to get one-note about it.)
2) For those who survive being possessed, should there be any long-term physical effects (arthritis, weakening of connective tissue, bone issues, etc.), or should it depend on how much and how long their body was altered by the monster?
3) Saul, an insecure transguy, is possessed by one of these monsters. (This monster is different from the others in that it's fully sentient but evil. It demands blood.) The sword wraps its tentacle vines around him, fusing with his flesh, to make him taller and beefier. (Some joints probably get dislocated though.) Would it make more sense for Saul to be stuck in this state until he can rid himself of the possession entirely, or could he slip out of his transformation depending on his sense of self, emotions, etc.? (The latter could be interesting narratively since some people knew Saul before his possession and thought that he had dissappeared or died. Saul and the sword are also in an abusive relationship.)
4) Saul's transformed body could get gaunt and weak when the sword wants more blood but Saul refuses or can't discreetly kill. Should I run with that?
5) I want Saul to get access to magical testosterone later in the story, so he can transition in a much healthier way. Does it make more sense for a testosterone spell to be classified under healing/medicinal magic? (It'd be like a testosterone injection, and much of the healing magic reflects modern medicine.)
5) Ruth is in her mid-twenties (or older?), has been studying magic through books for years, can shape-shift parts of herself into a dragon, and has a magic-boosting pendant. One of her arms is permamently covered in scales though. Her background: She was a noble, which gave her access to books, and now leads a radical feminist cult determined to overthrow patriarchy. Many of the societies are stupidly patriarchical, but she is an extremist and also steals princesses. (She's "rescuing" them.) Should she shape-shift using just magic, or would it be more fitting if she too had help from a monster, given how much of a prevalent these monsters already are?
6) Would having Ruth shape-shift into a full-blown dragon be a bit much? (IDK I'm on the fence.) Maybe her cult could help her cast it, staying more in line with the rules of magic?
Sorry for making such a long post! Thank you for reading this far down and/or answering any of these questions!
Also, if you see any holes in my logic, feel free to tell me. 🙂