A family of aircraft with as far as possible commonality, so that a pilot could go from smallest to largest with ease. Flight decks and handling characteristics as similar as possible along with the rest of the systems, SOPs and operational philosophy.
A single aisle 190 seater B787.
The current B787 with the 8,9,10 versions is well placed.
The new B777X once sorted out will cover the top end of the market.
Whilst there will be places for niche aircraft such as supersonic, VLA and middle of the market, why bother ?