Apart from derailing the path to the "happy ending" of the original Black Company stories, I don't know why this would fall under "should not be". (Though I haven't actually read the other half of the crossover, I just know enough about it to recognize the name.)
Midway through, the combination of "Shadowjack" with "fire from the sky" and "great bolts of lightning" led me to think that perhaps the new element was the (Inner?) Senshi, more experienced and/or more competent than we usually see them...
...and in the first few lines of the story, the first thing that caught my eye was "Rebel", so when I realized this was actually a Black Company snippet my first thought was that it was an AU and the enemy was the "Rebel Alliance" - as in Star Wars.
I'm not sure whether either of those fall under "must be" or under "should not be", but they're each definitely one or the other.
Midway through, the combination of "Shadowjack" with "fire from the sky" and "great bolts of lightning" led me to think that perhaps the new element was the (Inner?) Senshi, more experienced and/or more competent than we usually see them...
...and in the first few lines of the story, the first thing that caught my eye was "Rebel", so when I realized this was actually a Black Company snippet my first thought was that it was an AU and the enemy was the "Rebel Alliance" - as in Star Wars.
I'm not sure whether either of those fall under "must be" or under "should not be", but they're each definitely one or the other.