For what it's worth to this old topic, that is what I'd be most interested in seeing - how Mustang and his people set about trying to fix past mistakes and build a better future for their country. I'm not saying I'd ever expect to get a new manga/anime based on this, just that if I had a choice that is what I'd be most interested in seeing, with the Elrics making occasional appearances too. But it seemed the point of FMA:B is that they basically get to settle down and live happy lives so that's why I feel like it would be more interesting now to focus on Mustang and the rehabilitation as it were of Amestris.
There are a lot of interesting potential storylines that could still be mined in that plus Roy Mustang and Riza were probably my favorite characters and not because I shipped them(I'd be perfectly happy if they never got together and just sort of stayed what they were), I just felt they were complex and interesting characters and was always interested in finding out more about them. Plus I really like the relationships between Mustang's "supporting cast" too.
Then you have all the intrigue and power plays that would still be happening due to what they are involved in politically, there is a lot of potential story there both in reviewing the past and in their present.