Shun has become the most powerful Shinigami in history. In fact, this happened quite a long while ago.
But now, with the offer to continue his endless career as a Shinigami -- to walk the path he has walked since my very first post with him even further -- also comes the time to move on from the world of Bleach Gotei. The Plague is behind is, and Shun will remain until a suitable, new Captain-Commander emerges to lead a post-crisis Seireitei.
But his story is done, and has been, for a good, long while. It's time to close the book on this final relic of BG past.
Shun has accepted a promotion to join decently exclusive club of individuals performing a job of dubious importance. However, prior to acceptance, Shun will continue to run the Soul Society until a proper successor can be found. In game terms, this means he's retiring into NPC status until someone is eligible and willing to replace him.
I haven't verified this myself, but I heard there was even shit as recently as him threatening to bring in Hazumi to deal with people who don't like Otto. Nothing forces you to pay attention like a "If you ignore or go against my my prestige level Shinigami gon' attack you." statement.