I think the effects deserve a theory thread of their own. If you have a theory about them that you want to share, you could write it as a reply to this post. I will be doing so myself, shortly. :)
I think the effects might be Sabitsuki's thoughts about what the future could hold for her - some of the different hopes and fears she has.
Some of the effects probably relate to her fear about what the disease could do to her. Corpse and Ghost seem quite obvious - she knows her disease could be terminal. Arm and Mono Eye might be fears about developing visible deformities. Television might be similar, a fear of her life becoming a public freakshow due to her disease - after all, when she uses the effect, her face is on the screen. The Slime effect is probably related to the slime monster in the sewers (it's found quite nearby); perhaps her disease has phases which involve producing a lot of mucus? :/
I think some of the effects reflect Sabitsuki's fear about potential medical treatments, and what result they might have on her. The Machine effect gives her artificial limbs, very similar to those that a couple of NPCs have. Those NPCs could be her memory of a girl she met in a hospital, who had been given mechanical limbs. It's noticeable that the machine girl NPCs pretty much completely ignore Sabitsuki, so she probably didn't strike up a friendship with the real machine girl like she did with Oreko. She could be afraid of becoming like her.
Likewise, I wonder whether the plant children in the hospital, and the Plant effect, are something to do with some sort of medical research program involving plants? As a child, Sabitsuki may have heard something about the research and misunderstood it, becoming scared that it might turn her into a plant. Plants often seem to be a bit sinister in the game, with some NPCs entangled and trapped in them.
Most of my thoughts about the Steel Pipe and Handgun effects are based on what is shown in v0.16 (although it's difficult to know how much to trust what we see in that version of the game!). I think it'd be better to sum up my thoughts on v0.16 in a different thread, but the Steel Pipe effect is pretty central to that special version of the game. In the 'normal' game, Sabitsuki finds the Handgun effect in the place where she had found the Steel Pipe effect in v0.16. I think she remembers that she had found some sort of powerful weapon there when she was a child, but the details have become hazy in her mind. As a child, a steel pipe seemed powerful, but as a young adult, when she thinks of a powerful weapon she pictures a handgun. She looks quite scared of the Handgun when she uses it; I think her memories around the weapon she originally found are a bit traumatic. Depending on exactly what really happened in v0.16, I think the Steel Pipe could represent either her fear of a relapse into violent psychosis, or a desire to protect herself or to get revenge on someone. I think the Handgun probably mostly relates to her desire for self-preservation, as she has had some scary things happen to her in the past.
The Tattoo and Black Hood effects, and maybe also the Uniform, might be her hopes for blending into a gang or a close group of friends. I've never quite worked out exactly what happened to her at school with Smile and the Kaibutsu, but clearly the Tattoo effect is related to Smile. The Uniform might relate to fear that she will become like Kaibutsu, or it could just reflect that she wants to have a normal life again and finish her education. Similarly, the Black Hood could be something to do with wanting to be in some sort of gang, or it could just mean that she wants to live inconspicuously and shut out the rest of the world (by pulling up the hood).
I think Psychedelic and Watering Can are connected to events or settings in Sabitsuki's past which she would like to revisit. She may have experimented with a psychotropic drug at a night club (represented by the psychedelic streets, where she finds the Psychedelic effect). It's a bit harder to imagine what the Watering Can could be about. My best guess is that it's something she associates with a nurse whom she was friends with at the hospital (and who presumably died, given that she appears as a ghost in the game).
The Cat and Broom effects are very unrealistic. These might just be escapism - perhaps she's fond of anime, and of Kiki's Delivery Service in particular, and these effects are her dreaming about being a cute anime character instead of a troubled invalid. The Headphones effect may be another form of escapism. Sabitsuki does seem to like music, if the Neon World is anything to go by, and the headphones could indicate she'd rather just plug herself into music and not think about the future. The fact that she is wearing the headphones in the ending sequence might connect to this.
I'm not too sure about the Whistle effect. It gives you a bit of control over the movement of other NPCs, so I think it's something to do with Sabitsuki wishing to have more control over her life. It might also indicate that she wants more attention from those around her. It's found in a room with black-haired girls, whom I think might represent her dead mother; the loss of her mother would have left her feeling lonely and isolated, and she might feel like the world ignores her and her needs - hence she imagines the Whistle and the control it gives her over others around her.
The Diving Helmet effect is obviously connected to her memories of Oreko. She might worry that she will meet the same fate as Oreko (I think what's shown in the Dead Hole area indicates that she died), or she might simply be hoping to meet her again. I might be able to have a better guess if I had any idea why Oreko is always connected with diving helmets! One possibility is that Oreko had some sort of contagious disease which led to her needing to wear a helmet to avoid infecting others, and Sabitsuki is now worried of ending up in the same sort of position - but it doesn't seem very credible.
The other three effects are all connected to the gas mask maids. Tumbling Doll is likely to be to do with Sabitsuki being frightened that they might one day dismember her to keep her under control - and this fear is justified, as that's exactly what happens in the game's true ending! The Gas Mask effect could indicate that Sabitsuki thinks she could become one of the maids if her disease is brought under control. Alternatively, maybe she thinks she will one day need to wear one, to avoid being infected by other disease suffers or to avoid infecting people herself.
I'm not sure what the Dress is about. It does seem to be connected to the maids - you can make them bow if you curtsey to them while you are wearing it - but it's not obvious why. This might be a dress that Sabitsuki saw at a funeral (perhaps her mother's funeral). Maybe one of the maids brought Sabitsuki to the funeral, and was wearing that dress?
Some of these are a bit of a stretch. ;)