Will the element that Tony Stark created in Iron Man 2 pay off in Avengers 4? If so, how?
Well, it's a theory of mine that the element is actually a faux-Space Stone. Tony made the element from notes his father made while studying the original Space Stone. That is also why Loki could not mind control him like the others. Each Infinity Stone offers some resistance to the direct effects of the other Stones. When Thanos' attacks on Stark were all indirect attacks of the Stones (like throwing the moon at Stark). The one direct attack (shooting the Power Stone energy bolt at Stark), Stark blocked without ill effects. Thanos also didn't bother using the Gauntlet on other direct attacks, like he did against Banner and Rhodes because he knew the attacks would not work.
My theory is that Stark replicates that research to create some ability to resist the other Stones.
Since the element was synthesized from Howard's notes on the Tesseract, and the Tesseract was a housing that could contain and control an Infinity Stone.... Possibly. It certainly has potential as a plot device. At least, it's a fascinating theory to think about.
However, whether they go there in the next movie, and exactly how, who knows? Be almost a shame if it didn't play some part, since they actually made a point of showing - though not saying - that it is related to the Tesseract. That similarity saved Tony from control by Loki via the staff/Mind Stone in Avengers. It really should be revisited.
My best guess for how, would be in making some sort of containment unit so that a stone or multiple stones could be utilized.
Oh that element. What a stupid plot device that was. Stark's father developed the very element his son would need 30 years later to save himself from the arc reactor he invented. Makes Zemo's plan seem reasonable. Anyways, since it is what he uses to power his tech, so long as his tech has something to do with defeating Thanos then yes, it will pay off.