One popular trope is the 'Genius Rivalry'. Your OC can be another brilliant mind who either clashes or collaborates with Tony Stark. It adds a lot of tension and interesting exchanges. Another is the 'Underdog' trope. Maybe your OC is a new recruit to the Avengers with no real super - powers but a lot of heart and determination. They have to prove themselves to Tony and the others. 'Love Interest' is also common. Your OC could fall in love with Tony, or vice versa, which can lead to some sweet and sometimes angsty moments.
In Avengers Tony Stark OC fanfiction, 'Redemption' is a great trope. Maybe your OC has a dark past and is trying to make amends with the help of Tony and the Avengers. 'Team - up Against the Odds' is another. When all seems lost, your OC and Tony team up in a way no one expected to defeat a powerful enemy. Also, the 'Mysterious Newcomer' trope is popular. Your OC arrives on the scene with little known about them, and everyone, including Tony, is intrigued to find out more.
Well, there's the 'Secret Identity' trope. Your OC might be hiding a part of themselves from Tony and the Avengers for some reason. It could be a secret power or a past connection to a villain. 'The Mentor - Mentee' relationship is also popular. Tony could take your OC under his wing, teaching them about technology and hero - ism. Or, your OC could be the one teaching Tony something new. And then there's the 'Fish Out of Water' trope. If your OC is from a different dimension or time period, seeing them adjust to the Avengers' world can be really fun.