They could go on a mission to stop a group of tech - thieves. Kid Tony Stark uses his inventions to track the thieves, while the Winter Soldier uses his combat skills to take them down. Another adventure could be exploring an abandoned Hydra base. The Winter Soldier has some knowledge of the layout, and Tony's scientific expertise helps them understand and deal with any hidden traps or dangerous experiments left behind.
In an alternate universe, their interaction could be really interesting. Kid Tony Stark, with his brilliant mind even at a young age, might be both intrigued and a bit intimidated by the Winter Soldier. He'd probably be eager to figure out the technology behind the Winter Soldier's enhancements, while the Winter Soldier could see in Tony a potential for good or bad, depending on how his powers are used. Their dynamic could start off as wary but gradually turn into a begrudging respect as they realize they both have their own moral compasses.
Another theme is the power of technology. Tony is a technological genius, and his tech often comes into play when dealing with Bucky. Sometimes it's about fixing Bucky's arm, other times it's about using tech to help Bucky deal with his mental programming. In some fics, they might even create new tech together, which shows how their skills can combine.
One great Tony Stark Winter Soldier fanfiction is 'The Unlikely Alliance'. It shows an interesting scenario where Tony and Bucky have to work together against a common enemy. The story is full of action and also explores their complex relationship.
There's also 'Bucky's Vow'. In this fanfiction, Bucky feels a strong sense of protectiveness towards Tony from the start. It delves deep into their characters, showing how Bucky overcomes his past traumas to be there for Tony. The chemistry between them is really well - written.
A key element is often Bucky's past as the Winter Soldier. His training and instincts play a big role in how he protects Tony. For example, he can sense danger before others and uses his combat skills to keep Tony safe.
It could also be a new villain who has somehow managed to hack into the Winter Soldier's programming and redirect his mission towards kidnapping Tony. This villain might be someone who wants to create chaos between the Avengers by pitting the Winter Soldier against Tony. They stay in the shadows, pulling the strings while the Winter Soldier and Tony are caught in the middle.
One possible plot could be that the Winter Soldier is under some mind control still and is made to believe Tony is a threat to a certain organization. So, he kidnaps Tony to bring him to his handlers. But during the journey, some of the Winter Soldier's old memories start to surface, which makes him question his mission.
I like 'Stark's Little Spider'. In this, Peter is still very much a kid, and Tony's fatherly side really shines through. There are a lot of fun scenes where they build gadgets together, and the story also has a great plot about saving the city from a new threat.