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 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.
Maybe it's a tale where Howard Stark is somehow able to observe Iron Man's adventures from a hidden vantage point, and his reactions shape the story. Or it could be a story set in a parallel universe where Howard Stark has a front-row seat to all of Iron Man's heroic deeds.
One possible plot could be that Iron Man, Tony Stark, travels back in time and meets his father Howard Stark. He could try to prevent some of the events that led to Howard's death and in the process, learn more about his father's early inventions and ideas. Maybe they collaborate on a new piece of technology that changes the future of the Stark family.
Yes, it's possible that Tony Stark, as a character, could be the subject of Avengers fanfiction that he might 'watch' in the sense of being a character within the fictional scenarios written by fans. However, in the official Marvel universe, there's no indication of him actually reading Avengers fanfiction.
He's often shocked and intrigued. Fanfiction might depict him as both wary and excited about the unknown possibilities of the future.
He might be amazed at the technological advancements. In fanfiction, Howard Stark is a brilliant inventor in his own time. Seeing the future tech like advanced AI and space travel would both astonish and inspire him. He could start thinking of ways to adapt those ideas to his era, or he might feel a bit of envy for not being able to see it all in reality.
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.