Well, Howard Stark might have a range of reactions. He could be in awe of the advancements, or he might feel a bit left behind. Maybe he'd also have some fatherly pride and a desire to have been more involved.
Iron Man and Howard Stark would likely start with a bit of shock. Howard might be amazed at the technology Tony has created, and Tony would be in awe of seeing his father alive (if it's that kind of scenario). They'd probably exchange some witty remarks, with Howard being a bit more old - fashioned in his speech. Then they'd start discussing technology, with Howard offering some of his classic insights and Tony showing off his modern advancements.
There's no official indication in the main Marvel canon that Howard Stark reads Iron Man fanfiction. Howard was a brilliant inventor and businessman, mainly focused on his scientific and entrepreneurial pursuits. Fanfiction is a modern and often fan - created concept that might not have been part of his world view. However, in some alternate universes or fan - speculations, it could be possible, but still not a common or established idea.
He's often shocked and intrigued. Fanfiction might depict him as both wary and excited about the unknown possibilities of the future.
There could also be a scenario where the Avengers travel back in time to seek Howard Stark's help. Maybe they need his unique insights on some ancient technology or a threat that has roots in his era. So they go back in time and encounter him.
Some fanfics depict Howard Stark as having unexpected encounters with the Avengers, perhaps during time travel or alternate universes, leading to interesting plot developments and character dynamics.
Steve is usually shocked and conflicted. He might question a lot of things and have mixed emotions.
In many fanfictions, they have heart-to-heart conversations about family and legacy.
The Avengers would likely be filled with rage. Captain America, for instance, would immediately charge at Thanos, not caring about the danger. His sense of justice and loyalty to his friend would drive him to seek revenge right away.
He might be confused at first. Howard Stark was from a different era and fanfiction was not a thing during his time. But if he understood the concept, he might be flattered that his son's story had inspired so much creativity among the fans. He could also be critical as he was a perfectionist, and might not like some of the liberties taken in the fan - created stories.
He might be shocked at first. Just imagine seeing things that haven't happened yet. It's quite a mind - blowing experience.