What sort of age group do you have in mind, and what features do you think would be appropriate that aren't offered by the existing services?

The eldest, sixish. It's not blogging in the normal sense, rather a record of her creative writings that she can add to over time for her own self-validation and fun. Short pieces; the odd drawing, that sort of thing. Tumblr would be great if I could host it myself and control the privacy aspect better...

...and all this is assuming though, that such a thing would be attractive to her. I suspect so, though I have not tested it.