Okay, so I'm just going to post some of the limitations I presented to Yog, and see if they make sense to him and everyone else, so we can work this out before it even becomes a problem:
">This is one power that I think could be upgraded later, but you say that Jack can melt metal. This is of course certainly possible, but metal is a huge heat conductor. When it absorbs heat it immediately disperses the heat throughout. This would mean that Jack would have to apply his heat to small areas holding it there so as the area may reach it's ignition temperature. This along with surface area and exposure to air is why pots on a stove don't burn when you're cooking. A pot alone can handle at least over temperatures of 375 degrees Fahrenheit (190 Celsius) as this is the lowest average temperature of a stove on high. In order for Jack to even soften steel, a metal used commonly in architecture, he'd be using temperatures of around 1000 degrees Fahrenheit (538 Celsius), and to melt it would be a whopping 2,500 degrees Fahrenheit. I think that for now, Jack's flames should be at a much lower range, 450 would be your best bet as around this point their would be combustion from the oil he uses. While this wouldn't allow him to melt down metal with ease, it would allow for growth and potential, and would end realistically without people getting injured.
>Another thing you mention is Jack's ability to poison others if the oil gets in their bloodstream. You didn't really specify how this poison works, so I did a bit of research. Typically, yes oils are pretty hazardous, and can even cause death in the most severe cases, which would probably mean that Jack is intentionally trying to actively poison people, a deed that could also harm his teammates. However, it's much more common for oil to cause digestive problems like diarrhea or other issues like irritation, nausea, and vomiting. Also, I'm not sure how the oil get's into the body, unless actively ingested. Does it perform osmosis, or do you just mean the burning of the oil getting into the lungs? With either case, it'd be a risk to allies and civilians I propose that this move be for the most part cut. Sure if someone eats the oil, then the effects would make sense but inhaling the oil would only do meaningful damage if the exposure time was over a long period, and even then simply removing oneself from the polluted area would result in improved lung functions.
> He definitely needs more weaknesses. For one, while water definitely does make oil fires roar, it does eventually sizzle them out, so a huge weakness for Jack has to be water. Ice is a bit different, as it can be melted into a non-manipulative state if the user can't also control water. Smothering Jack's flames would also be a means of stopping his fire, without oxygen Jack's powers simply cannot work, because combustion requires it. Of course like all other members of the team, Jack's powers would be restrained by anti-titan gear which temporarily deactivates the titan gene. Overall I think adding these in would balance out the character, as having fire powers already makes him one of the strongest on the team.