Mall owners are still grappling with how to fill those spaces being vacated by department store chains like Sears and J.C. Penney.

It's no easy task and remains one of the biggest questions in the retail real estate industry today, Dirk Aulabaugh, managing director of Green Street Advisors' advisory and consulting group, told CNBC at the annual ICSC RECon event held in Las Vegas this week. The fact of the matter is, filling in the gaps takes time, years even, he said. And there aren't "cookie cutter" solutions.

Still, mall owners need to be on their toes now more than ever, especially in light of a potential asset sell-off at Sears and an unexpected CEO departure at J.C. Penney. CNBC reported earlier this year that 2018 is set to be a record year with respect to the amount of retail square footage going back on the market.

Many industry experts told CNBC at RECon that some department store chains are headed to extinction, while Nordstrom remains the so-called belle of the ball in the mall space — keeping a slimmer real estate portfolio, signing deals with e-commerce brands like Allbirds and maintaining top-notch customer service.

"In the next five years, we expect shoppers' increased emphasis on experiential retail to push retailers … as they compete," said James Cook, director of retail research for JLL. Landlords want those companies that will draw shoppers in with a personalized and "human" experience, he said. Topping that list today are companies like Apple and Ulta Beauty.

The divide was suggested even in the booths at RECon. T.J. Maxx and Homegoods owner TJX, Dollar General, Five Below and CVS had a prominent presence and there were more restaurant and hotel operators than there was in past years, attendees said.

Amid all the chatter about Bon-Ton's and Toys R Us' recent and ongoing liquidations, the mood at RECon this year was much more upbeat than past years, when developers and retailers alike were still crawling their way back out from the damage done during the Great Recession, Greg Maloney, head of JLL's Americas retail business, told CNBC.

"The sky is the limit for what you can do" with dark space at the mall or within shopping centers, he said. "You don't have to fill retail with retail anymore."

Apartment complexes and hotels were two of the new uses that some of the biggest retail landlords had top of mind this week.

Simon, a top-tier U.S. mall owner, announced during the show its plans to open at least five Marriott International hotels at its properties over the next several years.

PREIT, which has a smaller portfolio of malls primarily in the Northeast, said earlier this week that it would be looking to add as many as 7,000 residential units and 3,000 hotel units across a dozen properties in the coming years, too.

"Our business is requiring new creativity that's never been required before," PREIT CEO Joe Coradino told CNBC. The marketplace today, especially with younger generations of shoppers, is seeking centers where people can live, work, shop and dine, he said.

Coradino, along with other mall REIT CEOs at the show, agreed that more malls are transitioning to "mixed use" spaces. Fitness operators, spa service providers and other entertainment venue owners also flooded the Las Vegas Convention Center's halls this week.

"Experience is not just food," Deborah Weinswig, founder and CEO of Coresight Research, told CNBC.

Sure, mall owners are looking to add more food and beverage options at their properties, along with remodeling outdated food courts, but there are plenty of other "experiential tenants" to fill old department store boxes. She said she was most impressed with the amount of health and wellness initiatives being talked about at the conference this year — a category that isn't easily disrupted by Amazon.

Gregg Knoop, chief development officer of children's theme park chain KidZania, was seen making the rounds, meeting with U.S. mall and property owners like Seritage, a real estate investment trust looking specifically for ways to redevelop the Sears stores in its portfolio. (During the conference, Seritage signed a deal with gym owner Equinox to fill an old Sears store.)

"I've walked away from this conference with more inspiration than I have in a while," Weinswig said. "Landlords and their tenants are finally working together."