Lots of good thoughts there. Definitely going with the sketch first. I was considering making the map of the 'pre collapse' world and then working from there. I think you're right-on RE water levels. I had envisioned the former ocean/sea beds as new land...also presenting some interesting settings...lots of sunken ships dotting the landscape etc. Anyway, as you suggest, I'd like to have a general world map, but I'll only be detailing the areas that are 'in play' so to speak.
I came across a thread outlining some ideas for mapping out continental plates which I think would be useful, especially considering mapping those "long parallel mountain chains" where tectonic plates converge and diverge.
Thanks for the 2 cents. I should have a rough sketch up in a few days.