watchersonthewall.com Tons of stuff on there, including photos of character interactions from the new season.
I think it would be cool to see one follow a Maester going to Old Town and progressing if done right. And maybe that troupe of actors that Arya joined briefly. Just for insights into some new regions.
I think it all depends on what format they do this stuff in. Like a two hour special of Aegon the Conquerer might be cool, but then a whole mini series would be needed to do something like Robert's Rebellion.
Yeah I don't need them them do a whole new series like Game of Thrones...I like the idea of a 2 hour HBO movie for something, and then maybe run an AHS style show where they spend a season on a new story from the world. They're never gonna get the...what, 7 million per episode?...viewers for all of these things, but they've done such a good job with GOT that I think the people that are into the world will still watch. Probably could swing 1-2 million viewers just based off of that.
I think these would have to be a mini-series or a movie. Robert's Rebellion or a Targaryen story are the obvious/likely ones to start with, but I really hope that they focus on stories in new locations - @theagentcoma idea of Valyria before the doom would be awesome.
I thought either a mini series or just a movie or something on the origin of the Children of the Forest and/or the White Walkers would be cool (something with all new characters and actors). With Robert's Rebellion, you'll get cameos of some of the major characters from Game of Thrones so that seems like the way to go. Really, I'm just hoping if we get the rebellion, they make use out of characters they pretty much wasted in Game of Thrones (Ser Arthur Dayne, Barristan Selmy).
Personally would love to see Jorah's story told, from being exiled to watching over the Targaryen children and everything in between. But I don't believe there is enough there for a full show. A great concept I think would be wonderful is having a spin off that tells these stories in one season, with each being new every year like AHS....though obviously better than that abysmal show.
Or to take it even further... Do a mini series like Black Mirror where each episode is a new story (and varies in length). At first I didn't think something like that would work, but imagine Game of Thrones if there weren't twenty story lines going on in each episode. Each character probably only gets an hour of screen time per season anyway.