ETFs, of course, are becoming ever more popular as investment options in 401(k) plans.

Charles Schwab launched an ETF 401(k) back in 2014—and the option appeals particularly to millennials, with 70 percent of participants in that age group saying ETFs will be the core investment type in their portfolio in the future.

And the reach of managed accounts has grown, too, with the addition—also last year—of a Schwab managed account option for 401(k)s aimed at plan sponsor RIAs.

The Schwab managed account platform gives plan advisors the ability to develop customized saving solutions into which sponsors can default participants.

It also allows advisors the ability to design open-architecture portfolio solutions while containing costs through such options as—you guessed it, ETFs—and also Collective Investment Trusts.

That drive toward customization is evident in Stadion's approach, too. According to Stadion, the StoryLine platform "recognizes every sponsor and participant as unique."

It devises default options for each plan sponsor, dependent on the individual plan's makeup.

From there it encourages employees "to further define their individual investment paths based on personal risk profiles, expectations and goals," and allows the inclusion of outside and spousal assets to make the process more comprehensive.