Toyota Motor has a challenge: How to heed President Akio Toyoda’s insistence on heart-racing design for the next Camry and Prius models without jeopardizing their mass-market appeal.

Getting it wrong risks knocking Camry from its perch as the best-selling U.S. car, a title held for 12 consecutive years, and seeing Prius, which sells more than 200,000 units annually in the U.S., eclipsed by newer hybrid vehicles. While next-generation versions of both cars are in the works, Toyota executives haven’t provided many details.

The aim for the next Camry is a “more emotional, more impactful design,” Kevin Hunter, head of Toyota’s U.S. design studio, said in an interview at the North American International Auto Show in Detroit this week. “Camry’s taken some hits on styling, but it’s still selling well. But we need to create better design for Camry in the future.”

Toyoda, grandson of the company’s founder, is pushing an overhaul of vehicles with an emphasis on “waku-doki” design, shorthand for the Japanese phrase for heart-racing qualities. That would break from the current styling of the Camry and Prius, which together accounted for about a third of the Toyota City, Japan-based company’s U.S. sales volume last year.

“Finding the right balance is going to be very tricky” for the Camry, said Jack Nerad, analyst and executive editor at Kelley Blue Book in Irvine, California. “Most consumers like good-looking cars, they don’t want to drive appliances.”

Because Camry’s customer base, which Toyota estimates is 5 million owners, is so large, there are limits to how far the company can go in changing the design, Kazuo Ohara, head of Toyota’s U.S. sales unit, said.

“I would not go so far as saying we could be adventurous, but at least more aggressive,” he said in an interview in Detroit this week. With the current Camry, “we were probably a little bit too conservative.”

The emphasis on the next version of the car is enhanced interior packaging and materials and more “emotional” exterior looks, Ohara said, without elaborating on specific details. The company isn’t ready to say when a new Camry or Prius will go on sale, the executives said.

U.S. Camry sales were 408,484 in 2013, topping newer sedans, including Honda Motor Co.’s Accord, Nissan Motor Co.’s Altima and Ford Motor Co.’s Fusion, with faster-growing sales. The Accord is the No. 2 car, with 366,678 units sold last year, according to a Honda statement this month.

Even with Hyundai Motor Co. adding a revamped Sonata and Chrysler Group LLC releasing a new 200 model this year, Toyota, the world’s largest automaker, forecasts that Camry will maintain its lead and 400,000-unit volume for a 13th straight year, Bob Carter, Toyota’s U.S. senior vice president, said in an interview.

The Prius also faces competition. The world’s iconic hybrid is losing the one-of-a-kind status it held into the mid-2000s as the most advanced electric-drive auto in the U.S. Rival hybrids, plug-ins and all-electric cars, such as Nissan Motor Co.’s Leaf, General Motors Co.’s Volt and Tesla Motors Inc.’s Model S, are all alternatives to Prius.

“In the beginning Prius was the pioneer of the segment,” Jim Lentz, Toyota’s North American chief executive officer, said in an interview in Detroit this week. “As we put the powertrain into the Camry, it put hybrids into the mainstream. Prius did its job in creating hybrid as a mainstream option.”

The styling challenge for a new Prius isn’t as tough as for Camry, Nerad said. That’s because buyers “are very loyal to the name and more likely to accept something different,” he said.

As Toyota’s designers refine the look and layout of the next Prius, they said they will need to retain the car’s triangular profile for maximum aerodynamic efficiency and top-level fuel economy.

“The next Prius is in a difficult location in the market because hybrids already have become a fundamental technology,” Tokuo Fukuichi, a Toyota senior managing officer and head of global design, said in an interview in Detroit.

“We have to try to make a ‘future feeling’ and ‘best in the world’” fuel economy, Fukuichi said. “If we couldn’t get the best mileage, we don’t need a next Prius.” (Bloomberg)