One challenge is maintaining the integrity of both universes. 'Mass Effect' has a very detailed and complex lore, and so does 'Along Came a Spider' in its own way. The writer has to make sure that when they combine elements, they don't break the established rules of either.
Well, one good aspect could be the unique crossovers it might feature. Combining 'Along Came a Spider' with the 'Mass Effect' universe can lead to interesting character interactions. For example, the different species in 'Mass Effect' might react in unexpected ways to the events or characters from 'Along Came a Spider'.
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.
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.
Since I'm not sure if this is a well - known novel, I can't really say what it's about. It could potentially be about a character who borrows something and then a spider comes into the story in some significant way, perhaps as a symbol or an actual part of the plot.
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.