Here is the light outline of what needs to be answered:
What kind of environment will it take place in?
I feel two biomes is a nice. Even in the first series, The three forest territories have notable differences and landmarks and the fourth is a plains
Will we use book clans? Or will we create our own?
I prefer our homebrewed clans but I would not mind the book clans. 
Who will be Clan Leader?
GM
Who will be our Medicine Cat?
Co-GM
Will any two-legs be near us?
Unless there is a interesting and not cliche plot hook to include them, nope.
How many other cats will there be besides up players? if any?
Den mothers, Kits, Kittypets and rogues