As investors flock to low-cost index funds that generate only modest revenue for fund companies, many asset managers are seeking new markets.

What several companies, including some of the biggest, have hit upon is a form of financial planning, “a service for which investors are still willing to pay” high fees “that active management once enjoyed,” Don Phillips, a managing director at Morningstar, said.

Fund companies have introduced an assortment of services to assist financial advisers, or to take the task off the advisers’ hands entirely. By Morningstar’s estimate, advisers have invested nearly $35 billion in model portfolios provided by big fund companies and other so-called turnkey asset management providers. That compares with $13.6 billion at the end of 2016.

The roughly two dozen turnkey portfolio providers include household names like Vanguard, State Street Global Advisors and Pimco, along with less well-known managers. Some of them, especially Vanguard and State Street, have been beneficiaries of the shift to index funds. But as Mr. Phillips suggested in a commentary in the company’s magazine, asset managers can reap lucrative fees from advisers who use their model portfolios well beyond the more meager ones earned from the E.T.F.s included in the portfolios.