Wow, thanks a lot! I actually got a nice idea now thanks to you, just by reading those ideas.
Don't go into it thinking that whichever power you pick, it's going to eventually become something stronger. What you see is what you get, for the most part. I do encourage people to adjust their abilities; I really like Hero Academia's way of limiting what they can do (e.g ice dude can't ice too much or he gets cold, shadow beak is strongest during the night, prince charming gets stomach aches if he shoots too many navel lasers), so don't be afraid to gimp or to empower yourself in those ways. Creativity is more important than being fair. So Iron Mimicry probably won't evolve into being anything more than that, but the strength of the iron, how long you can keep it up, stuff like that could improve. Maybe at the start, you can only turn into iron when you're not moving? Or when you're holding your breath? Enhanced Breath can go the other way. You could evolve it to be more than just being able to blow air well, your lung capacity (as someone mentioned) can be huge, you could yell really loud, stuff like that.
Right now, my plan is to just give you guys a couple stats (like perception, strength, agility, intelligence) to cement your characters' abilities. For example, if something shifty is going on, the person with the highest perception score should be the one to notice something is going on. It's just to make it easier to compare people. I was pondering whether to give out points in a pool (where you divide among stats however you want) or from a scale (where you can give yourself how many points you want). The first way is more logical and how we usually do it — if you're all students, you should be pretty equal, just better at different things. But the other way is interesting too; if we have Mr. Equal, with 5 in every stat, fight against Mr. Balanced, who has 10 in every stat, we know they've both divided their points equally, but we also know how strong they are in comparison to each other. If we chose from a pool, they would both appear to be as strong. If we make an RP in a more chaotic setting, like in a prison, we might not want every character to be equal. Let me know what you think. We could also achieve this effect by giving people the option of spending less stat points in order to evolve their power or something like that — but in that case, people are still equal. Now some people just have more powerful powers than others while being physically weaker (which, since it's random, is already the case).
As for your power, personally I think it's much more interesting to think of it as the power to make imaginative friends come to life. Exchanging that piece of quirkiness (where you could make your character a lonely kid or a schizofrenic) for the power to just 'make anything you imagine appear' seems boring. I don't want you guys to just create a cool character like we usually do, and then slap a cool power on it. I want you to create the most creative power, and then base a creative character around it. Make it feel alive, as if both the power and the character came from the same idea. Don't make a cool assassin with dead parents that wants to avenge them with the power to shoot fire from his finger tips. Make an assassin who can see his dead parents as if they were alive, or a fire finger tip:er who is also a pyromaniac.