I can't NOT get in on this after seeing this thread!
So...
One of the issues we run into with this is that the children of Harry's friends - for the most part - are years apart from Maggie. So, I'm going to drop that idea all together and go with completely non team Dresden related ideas... other than the secret son of Marcone. That idea is just too amazing, especially if he's actually the Xander of the group because Marcone is trying to keep him away from the dark side of business until he's older. Protecting children is one of Marcone's goals, after all.
For a heavy... Troll girl has already been done with Meryl, so I'm going to avoid beings of fairy origin. Perhaps the scion of one of the small "d" dragons might be interesting. Someone who still has the ability to switch between human form and claws, wings, scales, fire-breath galore! But, who is just a whelp, a baby of their kind who can't do much else. Alternatively, we've already been given the Heavy in the form of Mouse. See "Mouse fights a van" in Proven Guilty. Or, maybe the two of them develop an almost Gimly and legolas relationship.
Another character that might be fun, if we're adding a feline presence to the team might be a Neko. Most of what we have seen so far is based in Western mythology and it might be fun to see a bit of what the eastern mythical creatures are up to. Are they fighting the Outsiders as well? Or, are they trying to deal with other problems? Yes, a child probably wouldn't let us see much of this, but they might provide a glimpse. Alternatively, they might provide nightmare fuel in the question of: Are Neko actually the changelings of Malks who have had children with humans?
Alternatively, the baby dragon could play the feline role, as the tendency in fiction is to depict them similarly to cats...
I do like the idea of Maggie making friends with the pretty and popular girl who begins as her arch nemesis... but the pool of possible stereotyped origins is rather slim. Sidhe of some kind? White Court? A sylph might be fun, providing the team with some air support(unless the dragon isn't too young to fly yet).
Another magic user or two would be fitting, I think. Harry does have a number of wizard allies alongside his closer friends. Perhaps the kid of a local sorcerer or paranet member who has a lot of promise when it comes to magic? Someone who's skills complement or contrast with Maggie's(if she's a potential wizard). It might also be cool to have someone like Abby from the Ordo, with future sight powers. Might make a fun "super awkward smart kid." Alternatively, someone with Casandra's Tears or - in general - some other divination-based ability.
...There are too many fun ideas.
Mostly though, I'm hoping for things we haven't seen before in large quantity. We've seen a lot of the vampires and the fairies, and I doubt we're going to get a twelve-year-old knight of the cross(unless Maggie sneaks out one night with Amorachious). What haven't we seen a lot of? That's what I want to see.
