what prevent one to setup a group of nodes, with system clocks set a few seconds after your genesis block timestamp, starting to mine it right after the blockchain data was published ?
Nothing prevents that. But at least everyone can do something like that. So it's "fair".
EDIT : or ... will the genesis block data be published one year in the future too ? (in what case, your "just" announce the coming-soon altcoin one year earlier?)
Yes, nobody can foresee which data will be included in the future genesis block. Not even me. As I said it will be the hash of the first bitcoin block for this day.