The risk will be too high. Most of the aircraft components are established, new features may be added, however, you don't expect everything to be new.
Besides, if the target day is 2020, it must be joking. 15 years for a commercial aircraft development. 15 years many thing will change, and with that long development period, the cost will be very very high. The right way to do this is to keep making small business jets, and have an R&D project to make a bigger plane. This project should be constantly monitored for cost and market needs. 15 years of commercial project inked as this moment is very very risky.