Neil Gaiman is on Tumblr and has answered A LOT of fan questions about season 2.

I'm going from memory and heavily paraphrasing, so look this up for yourself if you're still interested.

There was going to be a sequel to the Good Omens book, Gaiman and Pratchett had written a bunch of outlines and stuff for it, and it was going to be called 666 The Neighbour Of The Beast. Stuff happened and it was put on the back burner and never actually written, and then Sir Terry died and it couldn't ever be.

After GO got picked up for a tv adaptation of the og book, Neils' plan for if it was renewed, was always for GO to be 3 seasons, and to keep all of Pratchett's input.

Season 2 needed to happen the way it did as it's the world/character building and exposition necessary to set up the originally planned sequel- plus some incredibly well crafted fan service to keep it fresh. Neil told us it was going to be a gentle and romantic interlude (the bastard).

Season 3 apparently is going to be a lot more like season 1 as it's armageddon all over again, and this time they're serious. I think they're even bringing back jesus, who probably has more issues with god than Crowley by now.