It all depends on from whose point of view one is doing the "visioning".

From the airlines' point of view: as total miles continue to accumulate, seat liability will have to be limited... i.e. reduced. There will almost certainly be an increase in the two ways of accomplishing this: (1)more "non-flight" options for spending miles. This will help eat up the additional miles being earned by "non flight" spending.
(2) Continued watering down of the awards.. i.e., "award inflation", the raising of mileage requirements for more popular awards.

From the flyers' point of view, there will be increased pressure to: (1) make miles convertible, for instance from airline to hotel programs, or from one airline to a partner airline- as in airline alliances; (2) to extend the validity of miles... this is already well along; (3) to make the miles valid for more non-travel options... this corresponds to the airlines' vision, so is quite likely to happen.