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?

Many sites and places like this already exists, what makes you think everybody is going to ignore the other places that offer their analysis and 'predictions' and follow yours instead? Launch it if you want but I don't think it will have any impact on the price.