It could show Howard Stark's reactions to the events in Endgame. Maybe he'd be amazed at the technology used, or shocked at how things have changed in the future. He might also have some unique insights into the characters since he was involved in the early days of the MCU.
You can start by looking on popular fanfiction websites such as Archive of Our Own (AO3) or FanFiction.net. Just use the search bar on those sites and type in 'Howard Stark watches Endgame' to see if there are any relevant stories.
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.
One possibility is that the Avengers are facing a threat related to some old Hydra experiments that Howard Stark was once involved in. They meet him to gather information about the experiments, like the location of hidden files or the identities of key Hydra operatives from that time. Howard Stark, being the brilliant mind he is, joins forces with the Avengers to stop Hydra's resurgence.
Howard Stark could time travel in Avengers fanfiction by stumbling upon an ancient artifact with time-bending powers. Or maybe he's part of a secret experiment gone right (or wrong!), sending him hurtling through time. It depends on how creative the writer wants to be!
Well, it could be a story where Howard Stark plays a crucial role in helping the Avengers solve a major crisis. Or perhaps it's a lighthearted encounter where they have a friendly chat and share some insights.
It could be Iron Man, Tony Stark's father is Howard Stark. So Tony would have a very emotional and complex relationship in this meeting. He might be the main character as he gets to know his father in a different way and learn more about his past.