For large commercial aircraft engines fuel efficiency will always be a primary concern. History has shown that the long term trend is for fuel costs to increase. More fuel efficient engines reduce the economic impact of fuel price fluctuations on the operator. Another thing to consider are the much tighter emissions regulations being implemented for CO2, NOx, etc.
Improving commercial aircraft turbine engine efficiency can be achieved with higher bypass ratios, higher cycle pressure ratios, higher cycle temperatures, lower mechanical & thermal losses, and improved controls. The higher cost from using more complex engine systems to get better efficiency is almost always a good bargain for large commercial aircraft engines.
As barit1 noted, open rotor/propfan designs are likely the ultimate solution for high bypass ratios. They can involve some additional complexity, especially with counter-rotating configurations. The approach used by
GE on their UDF design eliminated the gearbox by coupling the fan blades directly to the OD of LPT stages. But the tradeoff was a more complicated LPT design in terms of stages, sealing, fan blade pitch control linkage, etc. and more compromise between the turbine blade and fan blade geometries.
There are existing large three stage turboprop/propfan engines, such as the
Rolls-Royce TP400-D6 and the
Progess D-27.