A Jedi with all the Infinity Stones would be an extremely powerful force. They could use the Time Stone to foresee any threats to the galaxy and prevent them before they happen. With the Mind Stone, they could potentially influence the minds of Sith Lords to turn them away from the dark side.
If Spider Man had all the Infinity Stones, he would be extremely powerful. He could use the Time Stone to reverse bad events or prevent disasters. The Power Stone might enhance his physical strength to an unimaginable level, making him even more capable in combat against powerful foes. And with the Mind Stone, he could perhaps gain greater wisdom and understanding of his enemies' plans.
The plot would be much more complex. Harry could easily defeat Voldemort with the combined power of the Infinity Stones. So the main conflict of the original story would be resolved very quickly.
In this type of fanfic, the Infinity Stones could have a very different effect in the Harry Potter universe. For example, the Time Stone could be used to reverse dark spells or resurrect fallen wizards. However, the Stones' power might also be limited by the magic rules of the wizarding world. The fanfic could delve into how the characters from both universes adapt to the presence of the Stones and how they use or misuse their power.
Well, it could be about the origin of the Infinity Stones. There could be a powerful being or a group of beings that created them for a specific purpose, like maintaining the balance of the universe. The story might then show how this intention got corrupted over time as the Stones became objects of desire and power struggles. There could be a backstory about how the Stones were scattered across the universe and how different civilizations came to know about their existence.
One of the best is 'The Infinity Gauntlet'. It's a classic that really delves into the power of the Infinity Stones and the epic battles that ensue when Thanos gets his hands on them. It shows how the heroes of the Marvel Universe come together to try and stop him from using the stones for his malevolent purposes.