Single engine Airliners? Not for a loong time.
There has been studies of single engine business jets, wich might be a good thing, but flying single engine in an airliner is something different.
Once there was a very likely scenario of loosing one engine (late piston, early jet age) and loosing an engine on a transatlantic flight was, well not common but hardly uncommon. Therefore the requirment for several engines. Now we consider the chance of loosing a single engine very remote, loosing two engines impossible (almost).
An aircraft has to be redundant and since no airliner can perform a all engine out glide safely (like say a cessna) I hardly think flying with only one engine (to start with) is an option.