To be perfectly honest, I don't know much about the Highlander canon. Hsu's mun has been giving me some pointers here & there. I know practically nothing about the Horsemen. My only thing with Cass being her Teacher is the timing. Quinn became an Immortal probably around 50AD in Norway, so I'm not sure how that lines up with Hsu's timeline. There's a definite storyline going around my head about Quinn & Hsu having a past, but I need to run that by his mun first. If she gives the go-ahead, having Cass around would definitely be a bonus!

By the way, how much do you know about Watchers? I know there's the whole Hsu's-On-The-Watcher-Warpath thing, but there are some "good" Watchers around, right?