The NFL is very much a league of trends. Teams obviously tweak their schemes as well as their personnel in trying to improve. With the pressure to win now, it's also no secret that other organizations attempt to duplicate successful changes.
An organization's ability to foresee these trends and counter them is critical. One trend to watch is the development of versatile safeties. Traditionally, free safeties excel in coverage but aren't great in run support, while strong safeties generally are stout against the run but lack ideal cover skills.
The movement toward safeties who can defend the run and the pass might have started with the evolution toward larger, stronger, faster receivers. Today's receivers generally possess a greater combination of size, strength and speed than those of 10-15 years ago, so corners have a harder time matching up. This placed a premium on shutdown corners and forced defenses to play more cover schemes that give the corners safety help over the top.