The plot might center around a web - like conspiracy (a nod to the 'Along Came a Spider' theme). Kara could be the muscle, Peter the brains and Donna the heart of the group. They start investigating something small but soon find out it's part of a much larger and more dangerous scheme.
Peter could be constantly in awe of Kara's powers, which might lead to some humorous exchanges. Donna, on the other hand, would be the one to ground them both. She would make sure that Kara doesn't go overboard with her super - heroics and that Peter doesn't get too lost in his thoughts. They would interact in a way that combines their different strengths and personalities.
It could start with a strange occurrence where spider - like creatures start appearing all over the city. Supergirl and Donna begin to investigate. As they dig deeper, they find out that there is a powerful force behind it all. They face many challenges along the way, like dealing with the webs that the spider - creatures create which can trap even Supergirl. Donna might use her combat skills to fight the smaller creatures while Supergirl takes on the bigger threats. And in the end, they discover the mastermind behind it all and save the city.
I'm not sure about specific fanfics with that exact title. But you can try searching on popular fanfic websites like Archive of Our Own or FanFiction.net. There might be some creative works there that combine these elements in an interesting way.
Power struggles are also quite common. There are usually other mages or magical beings that the spider mage has to contend with for power or resources. For example, in some fics, there are rival mages who want to steal the spider mage's unique powers.
I'm not sure specifically as I haven't read that particular fanfic. It could be about a spider - perhaps a spooky or mysterious one that arrives at midnight. Maybe it has elements of mystery, danger or a unique story related to the spider's appearance at that time.
One common theme could be friendship. They might explore how Peter, Donna, and Kara become friends and the adventures they have together. Another theme could be facing challenges as a group. For example, they could be up against a powerful enemy or trying to solve a difficult mystery.