Things to do
I've already shaken up my campaign world but letting the players go back into the past and help an ancient empire that once occupied the same geography as the current campaign import iron and steel technology. This allowed it to stay in power longer, changing several historical events including a deific war that brought down one of the three moons.
I've already made a new broad map, including moon-induced changes to geography (aka big honking craters where some cities used to be). This map covers the equivalent of the eastern half of the US. The landmass and major features such as mountains and large lakes remain the same, and many localities and kingdoms that weren't part of the old empire are recognizable.
Having the empire stick around longer also lets me indulge in some linguistic amusement. I have a penchant for using real languages as the basis for my campaign languages. The predominant language of the old campaign area was Cymraeg (Welsh), while a gypsy-like ethnic group called Vagistos had a language called Vagistol (Spanish) and the inhabitants of a nearby group of large islands of very similar racial stock spoken Catalan (Catalan... because I was too lazy to even file a serial number). This racial stock is descended from the ruling ethnic group of the old empire. Since this empire was farther spread and much more recently fallen, I had to give it a name and a pseudo-language.
Thus was invented the Empire of Qatal and the language Qatol. For words I decided to start lopping trailing syllables off and fiddle from there, as well do some consonant sleight-of-hand. Leading C became Q (pronounced like the German of Scottish ch) and Catalan became Qatal. I've also started reverse engineering significant place-names from my pseudo-Welsh sounds to the new style. So what was once the Duchy of Cymwr became the Kingdom of Qimur. The surviving cities had the Caer changed to Idad (from Spanish ciudad), so Caer Gwlad becomes Idad Gulad.
So on to the list:
Name cities and countries/kingdoms
Name towns the players are likely to go to or ask about
Name the new pantheon, establish cleric abilities and church roles
Create basic monk and bard rules to cover allies in cast
Create House Rule for utility skills to fill in gaps since Perform, Craft, Profession and some other fluff skills are all gone
Create guideline governing availability of Rituals
Create new commerce organization to replace the Temple of Paulic (in the old world, the Temple to the God of Wealth also functioned as a bank)
Decide how much debt the PCs still owe above
Determine trade route travel times
Set up place on new website for House Rules
crap... it seems like there was a lot more rattling around in my skull.