Should Apple's next killer product be the iCar?

Taking his lead from billionaire investor Carl Icahn, analyst Andaan Ahmad at German investment bank Berenberg has written an open letter to Apple chief executive Tim Cook and chairman Arthur Levinson. Ahmad calls on Apple (AAPL) to buy electric carmaker Tesla (TSLA).

The London-based Ahmad -- who says he's covered the tech industry for nearly 20 years -- argued in Friday's letter that Apple's shift into the auto sector could give the company the kind of revenue growth that won't be sustainable from just smartphones and other mobile devices over the longer-term.

The move could reignite the U.S. auto industry and would be a catalyst to accelerate the current transition to hybrid and electric vehicles.

Ahmad said the strength of the Apple brand and its history of "disrupting" industries are reasons why his bold plan could make sense.

And then there's Tesla chief Elon Musk, who Ahmad sees as an innovative presence like the late Steve Jobs. Many analysts and investors have worried that Apple has not been the same since Jobs passed away two years ago.

"You could strike up a partnership and obtain a new iconic partner to lead Apple's innovation drive," Ahmad wrote.

While noting his proposal will be ridiculed by some, Ahmad said that Apple needs an "out of the box" move into a new market. Otherwise, he thinks "the key debate will always be about your ability to sustain these abnormal margins in your iPhone business."

Related: Icahn demands $150 billion Apple buyback

This is the second piece of unsolicited advice Apple has received in the past week. Icahn issued an open letter to Tim Cook on Thursday and urged the company to buy back $150 billion of its own stock.

Icahn thinks Apple's stock is extremely undervalued and that the company should invest in its own shares right now.

Berenberg's Ahmad poked fun at Icahn in his proposal, saying he'd love to meet Cook and Levinson in London and promised not to tweet about their talk.

Apple, Tesla and Icahn weren't immediately available for comment.

Apple will release its fourth-quarter results after the closing bell Monday. Sales are expected to be up slightly from last year but analysts are forecasting that earnings were down again.

Although Apple's stock has rebounded sharply lately on hopes of strong iPhone sales, shares are still down year-to-date and are 25% below their all-time high from September 2012.

Tesla, on the other hand, has been a darling of Wall Street. Shares are up nearly 400% this year. The company's market value is now about $20 billion. The stock's meteoric rise has prompted Musk to say on more than one occasion that he thinks the stock has a higher price than it deserves.