Actually, I think it would be more interesting if Tony didn't design it.
What if Tony was severely depressed after Steve's death? Thus causing a sort of mental chemical imbalance. The Extremis notices this and decides to correct it by finding a way to make Tony happy, in this case, it creates a sort of hallucination of Steve that's not only like Steve, but is also more flirtatious/seductive.
Tony is smart enough to figure out that this Steve is a hallucination. However, the issue isn't actually figuring out how to stop the Extremis from doing this, it's if Tony 'wants' the Extremis to stop.
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What if Tony was severely depressed after Steve's death? Thus causing a sort of mental chemical imbalance. The Extremis notices this and decides to correct it by finding a way to make Tony happy, in this case, it creates a sort of hallucination of Steve that's not only like Steve, but is also more flirtatious/seductive.
Tony is smart enough to figure out that this Steve is a hallucination. However, the issue isn't actually figuring out how to stop the Extremis from doing this, it's if Tony 'wants' the Extremis to stop.