Yeah, that's pretty much the setting for him already. He's far away from Equestria and Princess Celestia avoids him like the plague. So that leaves the question, why would they confront him all of the sudden? He must form a threat of some sort, other than just the looming threat of him existing in the first place. But he's a dragon. Dragons are already hard for ponies to beat, and this one is feared among his own kind. He even has his own army of bird-brained wyverns. He's a massive, insanely powerful, experienced, smart elder. His scales have hardened immensely through battle and age and he has a slight bit of resistance to magical attacks due to earlier exposure.
Discord getting reformed was a blessing for my story, really, because now the ponies actually have some kind of advantage. Discord's ability to alter reality is definitely going to be needed to beat him.