Sure, Jane Thor is in the comics. Her character brought a fresh perspective and showed that anyone could be worthy of the Thor's power and responsibility.
Hulk is another character that could be popular. His brute strength and his unique relationship with Thor in Ragnarok are great fodder for fanfiction. Writers might explore his time on Sakaar in more detail, or his interactions with the Asgardians when he finally arrives in Asgard.
Love is a very common theme. Their relationship, whether it's the budding of their love or the challenges they face as a couple. Another theme is adventure, as they often go on quests together. And power, like how Thor's godly powers interact with Jane's scientific knowledge.
Love is a very common theme. Since their relationship is central in the movies, fanfictions often expand on their love story, exploring how it deepens and overcomes obstacles.