The market for U.S.-listed exchange-traded funds topped $2 trillion by the end of 2014, and the space seems like an unstoppable force. But is it?

“Are we getting perhaps closer to the point where ETFs as a category might be close to fully mature?” asked Michael Iachini, managing director of ETF research at Charles Schwab Investment Advisory. “Maybe not in 2015, but I don’t think that date is necessarily all that far off anymore.”

Iachini spoke today with two other panelists on a media conference call hosted by Charles Schwab that discussed the trends and outlook for ETFs in 2015. Iachini, who focused on potential growth outlets for ETFs, said he doesn’t expect the industry to shrink regarding asset size, but noted that unless new ideas are hatched and catch on it might be getting closer to the point of product consolidation.

“When you look at the ETF landscape there isn’t a lot of totally unexplored territory, although ETF providers are always searching for it and trying to find new products that investors might want,” Iachini said.

He touched upon the potential trend of non-transparent, actively managed ETF structures designed to avoid daily disclosures of portfolio holdings. Daily disclosures are a big bugaboo for traditional mutual fund companies who’ve been reluctant to roll out ETFs that mimic their fund strategies. Various fund companies have filed with the Securities and Exchange Commission for approval to launch non-transparent active strategies that don’t make daily disclosures, but with mixed results.

In October, the SEC nixed a proposal from Precidian Investments and BlackRock to list and trade non-transparent, actively managed ETFs based on a blind-trust format. But in November, the agency gave the green light for Eaton Vance to bring its exchange-traded managed funds (ETMFs) structure to market that employs a net asset value-based trading methodology. Later this year, the company plans to issue a suite of ETMFs under the NextShares name that mirror Eaton Vance’s existing mutual fund strategies.

Iachini doesn’t expect the non-transparent, actively managed ETF movement to have much impact––at least not this year. And unless they take off in a meaningful way beyond this year, “I think we’re probably closer to the year where the number of ETFs out there shrinks rather than grows tremendously,” he said.

Regarding asset growth, Iachini sees several potential growth areas including increased use among retail investors especially, but also among financial advisors. He also sees big potential for ETF use in the 401(k) market. “You look at the growth of the mutual fund industry and how closely it’s been tied to the growth of 401(k)s,” he said. “If ETFs ever become a common option in 401(k)s that could be another big avenue for growth, which ties back into that retail investor story.”

Heather Fischer, Schwab’s vice president and head of third-party ETF platform management, noted that in its role as one of the industry’s largest custodians of retail ETF assets, Schwab finished 2014 with $24.5 billion in inflows and more than $230 billion in total assets under management––the latter was up 18 percent versus the prior year.

She noted the company’s ETF OneSource platform of commission-free ETFs accounted for 43 percent of ETF flows last year at the company, and now comprise 16 percent of ETF assets held at Schwab.

The lone non-Schwab panelist, David Mazza, vice president and head of ETF investment strategy at State Street Global Advisors, said he’s focused on three distinct areas within ETFs this year––fixed income, sectors-based investing and the rise of smart-beta funds, which are a broad category based on indexes that aren’t market capitalization-weighted. He anticipates volatility in the fixed-income space in response to expectations of divergent monetary policies between the U.S. Federal Reserve and central banks in Europe and Japan. As such, he believes many investors will shift money from longer-duration to shorter-duration credit products to protect against rising rates.

Regarding sector investing, Mazza says beaten-down areas such as commodities and energy could become opportunities this year if valuations become attractive.

As for smart-beta, “it continues to be of interest to investors looking for new ways to bridge the gap between passive and active management,” Mazza said. “Opportunities exist not just for low-volatility or dividend investing [smart-beta strategies], but also for more multi-factor approaches.”

He also spotlighted another potentially interesting area for 2015––the hybrid asset class of convertible bonds. “In the quest to make portfolios more resilient to rising interest rates or make equity portfolios less volatile, convertible bonds fit into an in-between area that can potentially help reduce volatility from equities and change the dynamics of historical fixed-income portfolios,” Mazza said.