All new characters must be neutral or evil in alignment. Clerics and Paladins and Warlocks need to contact the DM ahead of time to work out details of deity, oath, patron, and so on. For each PHB race, imagine that race in the aftermath of the ultimate victory of good. Players will have lots of input here. PCs will start off knowing each other
Characters need to be created with one of these starting ability score sets. (link) or to use some kind of point-buy system.
Players also need to buy into the idea that the PCs will be evil characters who are still, for the most part, working together. Why? Because the other PCs are the only significant allies any of you will find in the known world. Backstab them or burn bridges, and you’re cut off and alone – basically fodder for glory-seeking Paladins to make a name on.
Starting characters have starting equipment – it’s hard to make a living in the world now that it’s been heroically redeemed. You’re down to your last few meals and coins, with nothing but what you’re carrying on your back.
Basic firearms as described in the DMG are available, though uncommon. Long distance travel is accomplished magically, including by powered airship. There are also pleasure balloons for the wealthy.