If I will launch a big site with BTC/USD rate forecast, will it be cool?
Lot of people will visit this site and see - if it says - buy bitcoins now! - they will buy and price will grow.
When it will turned to sell bitcoins - people will sell, and price will go down. How do you think, should I launch this?

There are sites which actually try and predict what the price will be in the next few hours. But nobody follows it based on that.
What you seem to be suggesting is a pump and dump group similar to altcoins, but that will not work for bitcoin at all
There are laws which prohibit a public price manipulation in stock markets, which are bigger than Bitcoin.
There are no laws which regulate bitcoin price manipulation, why are you sure that it will not work? It will, if somebody like me will do that. It will not work if usual webmasters will, without my abilities to attract millions on sites.