In many cases, Dragonite is good for the Soul Silver story. It has high stats, a wide movepool, and can handle various challenges. However, it also depends on your team composition and playstyle.
One possible plot could be that Dragonite gets separated from Iris during a dangerous adventure. Iris is desperate to find it and Dragonite is trying to make its way back to her. Along the way, Dragonite has to face various challenges and meets other Pokémon who either help or hinder its journey back to Iris.
You can try looking on fanfiction websites like FanFiction.net or Archive of Our Own. There are often many stories featuring different Pokémon and their trainers, so you might be able to find some great Dragonite and Iris fanfics there.
Yes, in many fanfictions, Ash's Charizard is depicted as extremely powerful. It has evolved over time and gained a lot of battle experience with Ash. Its strength is often magnified in fan-created stories to show its dominance in battles against various opponents.
In some fanfictions, Ash gets angry. He might be angry at the world, at fate, for taking his mom away. This anger could manifest in different ways, like being more reckless in battles or snapping at his friends when they try to comfort him. But deep down, it's just his way of dealing with the overwhelming pain of losing his mother.
A new, powerful villain could be the cause. This villain might target Ash's Pokémon and manage to steal them all away. Ash would be heartbroken not only because he lost his Pokémon but also because he feels he has failed to protect them.
Ash may become more mature. He starts to think more strategically about his battles and Pokémon training. His understanding of the Alola region's Pokémon and their abilities deepens.
It could make Ash more self - reflective. Releasing Charizard, a Pokémon he's so close to, would force him to think about his role as a trainer. He might become more cautious in future decisions about his Pokémon.