I could have been interested in a slayer school spin-off, but Dawn was never that interesting. In a way, it kind of was the point, for the Key to be unnoticeable.

I agree, she did a great job as Echo. The weak link was the pacing of the show. S1 was too slow, and then, they probably knew they wouldn't get past S2, so it felt like they stacked the content of 2 seasons into 1.