It wont be improvised, but it isn't decided yet, my plan is as follows:
session 1) check so all player are there, check so all the software works, etc. Then every player privately gives a few wish-lists and suggestions to me, then there is general brainstorming until we come up with an idea for a general setting that everyone likes.
Between session 1 and 2) I think up the details of the setting, various possibilities and impossibilities of character types, and the starting situation
session 2) Character creation (or specification, for those that had already decided some things), start roleplaying, both the players and the characters get to know each other and ideas go from fluffy possibilities to solid IC realities.
between session 2 and 3) I think up the actual plot, and plan out the adventure in actual written-down detail, and set as many non-chelovsikis-gun setting details as possible in stone.
session 3+) The actual adventure and plot starts, and if all goes well probably has more detail structure and depth than most.
The thing to note here is what happens betwen the sessions, something that happens automatically and even invoulenterly whenever I have a new and intreasting setting that is unexplored, and that I here try to turn to my advantage; that at every moment, in the bathroom, when going to sleep, on the bus, waiting for a computer program to load, etc. my brain will race at a gazillion kilometres per hour to obsess about the setting and iterate throughout every minor aspect of it generating ridiculously massive, deep, detailed and comprehensive data on just about everything. In fact, even in the unlikely even this process would fail, I still have dussins of massively detailed settings to draw from, generated long ago.