From the experience of last gen, pokemon lists with stats, traits and movelists will appear almost immediately.
Move data is probably the next most important thing, then traits, then items. Because these are the things that have the biggest impact on battling.
[edit] - the damage formula actually took ages to work out. But ultimately, it was very similar to the old one. In terms of actually playing the game it didnt really have a significant impact. Just in the creation of the simulator.. That sort of thing imo should be a low priority, because it isnt hugely important to be 100% accurate on, and because its really difficult to work out. Or at least this is my understanding..
Have a nice day.