A poster in the comments on RRH had the rather interesting theory that Boehner had intentionally asked Grimm to delay his resignation until the next scandal/next time it was necessary to distract headlines and deployed him now to get the pressure off of Scalise. Fascinating.
Anyway, most of the candidates have probably been listed in this thread: DA Dan Donovan seems to start off as the favorite for the GOP nomination, while Ds are probably hoping to convince former one-term Representative Mike McMahon to run. McMahon considered and then rejected running in 2012 and 2014, but there might be more pressure on him due to this being a special election.