I think having a character like Dawn be a Watcher would be a departure from the overwhelmingly male Watcher's Council representatives we saw on the show- she's clever, and I think the role would suit her, but as someone who's got to be a very similar age to the new Slayers, I think the dynamic between her and "her Slayer" would be very different to, say, Buffy and Giles, where there was very much a parent/ child dynamic.
I also wonder whether Buffy and Faith themselves may slip into the role of Watchers as they grow older (assuming that they do indeed grow older, which is much more likely with a multitude of Slayers). From what I understand, they're still very much active "in the field" in the comic books, and as they are still quite young and likely in peak physical condition, this is understandable. However, whilst they will always be stronger than the average human being, assuming that age will take it's toll on their strength/ flexibility, etc. then they're not always going to be stronger/ as strong as the vampires and demons they face. Would either of them slip into a more "Watcher-y" role as they grew older? I didn't think Buffy made a great leader in S7, but would it be different being responsible for the training and development of one or two specific Slayers, if she had a chance to bond with them the way she never did with the Potentials?