I wouldn't mind doing another simulwatch, not sure I have any good ideas for what to watch though.
I picked up the Bubblegum Crisis set recently since they added the option to get it without having to worry about customs. I'm not sure it's a good option since it's got fewer episodes than most series and they're (mostly) longer than standard episodes but I haven't seen it before and it might be interesting to watch along with both people that have seen it and those that haven't (assuming anyone else blind bought it and hasn't watched it yet). I don't think it's legally streaming anywhere though so that's probably going to be the biggest issue.
I'd be tempted to suggest Shirobako, although it hasn't been so long since it was first on (well, I guess it started a year ago now). I think it could be a good one for a simulwatch (and it's available on Crunchyroll) but maybe it's a bit too soon.
Natsume's Book of Friends/Natsume Yujin Cho could be interesting, although it's longer than Nadia if you watch all four seasons. I suppose it's the kind of show that maybe doesn't have such a wide appeal though, you really have to be up for some slow paced and thoughtful stuff. In a similar vein Mushi-shi could be fun but I don't think the first season is available streaming at the moment.
Space Dandy is only on Netflix just now right? I think it could be an interesting option since each episode is fairly different.
I suppose if we're talking about more episodic shows then Cowboy Bebop might be an obvious one. Maybe too obvious (and again, I don't think it's streaming).
I'll have to give it some thought, watching something I haven't seen would probably be more interesting but then that also makes it harder to tell which ones could be good options.