From bubble rumblings to increased volatility and everything in between, 2014 has been quite a year on the stock market. And in 2015, things could get even more interesting.

Based on shareholder letters and other writings from George Soros, Carl Icahn, Steve Mandel and other billionaire investors as well as data gathered by iBillionaire, here are four predictions for the year ahead.

1. U.S. Is the Best Place to Invest

Europe barely dodged recession in November, but the region is nowhere near out of the woods yet. In a column entitled Wake Up, Europe, billionaire George Soros addressed the issue, contending “the European Union in general and the Eurozone in particular lost their way after the financial crisis of 2008.”

In other words, Europe hasn’t bounced back. The United States, however, has.

In his third-quarter letter to Lone Pine Capital investors, Steve Mandel, that firm’s founder,  highlighted the pace of the U.S. stock market in 2014, driven by what he called a “triumvirate” of large-capitalization, healthy dividend players, management change/activist situations, and “big dream” companies.

Third Point LLC head Dan Loeb echoed Mandel’s U.S. optimism in his own Q3 letter to investors. “Going forward, we expect that the U.S. will remain the best place to invest,” he wrote. The billionaire’s letter also demonstrates waning optimism on Japan – a market Loeb was eager to attack just a year ago.

2. Icahn Says Apple Is Undervalued

In November, The Economist asked Carl Icahn to write 850 words advising investors on how to profit in 2015. His response? To follow him on Twitter (actually a pretty worthwhile endeavor) and, on a more serious note, look at Apple (AAPL).

The billionaire, who owns 53 million shares of Apple, believes that Apple is still greatly undervalued, and in the piece, he provided an explanation as for why: “The answer is a pervasive misunderstanding among investors and Wall Street analysts: they think Apple is a hardware company when in reality it’s a company that sells an entire ecosystem of hardware, software, and services.”

It’s worth noting that not everyone is as bullish on Apple as Icahn. In fact, during the third quarter of the year, a number of big-name investors reduced their positions in the company, including Ray Dalio, Julian Robertson, George Soros, David Einhorn and Leon Cooperman.  

Icahn also took advantage of his Economist platform to plug Icahn Enterprises (IEP)and tout his activist approach to investing – something that won’t likely be going away in 2015, either.

3. Amplified Activism

Speaking of activism, that’s another billionaire trend to look out for in 2015. In fact, many hedge fund heavyweights have already gotten the ball rolling on their next big activist pushes.

Bill Ackman raised eyebrows in November with the disclosure of an 8.3% stake in Pfizer spinoff Zoetis (ZTS). Within the regulatory filing are hints at a potential activist push, and Ackman dedicated an entire page of his Q3 letter to Pershing Square investors to the animal health company. In other words, just as his Allergan bid is winding down, he’s acquired his next target.

It looks as though Dan Loeb’s Dow Chemical (DOW) play could continue into the new year as well. The billionaire engaged in quite the campaign against the company, going as far as launching a website and video attacking it for “broken promises” in November. Though the two parties have signed a peace treaty for now, there’s no guarantee the story is over.

Also worth watching: Carl Icahn and Hertz (HTZ). The billionaire raised his stake in the car rental giant to 10.77% in November – a potential sign he’s not yet satisfied with the wins he’s already racked up on management changes and board seats.

4. eBay Value Unlocked

In late September, eBay (EBAY) surprised the market with the announcement of plans to spin off PayPal in 2015 – a move it spent much of 2014 trying to avoid. And since then, there’s been a lot of chatter as to what the transaction will imply.

Loeb expressed confidence in the split in his latest investor letter. “Following the spinoff, eBay/PayPal will offer two appealing growth, relative value, and capital return profiles for investors,” he wrote, contending eBay could be worth more than double its pre-split value.

In a November interview, Larry Robbins addressed the spinoff in a humorous manner, likening it to a divorce. “Once they are separated there is one company who will focus on spending and another company that will focus on payments,” he quipped. “Post-divorce the odds of getting lucky increases. The companies when separated will be more attractive to another suitor and could get into a beneficial takeover relationship.”

He also appealed to comedy to address concerns that Apple Pay could hurt PayPal. “Being scared that Apple entering mobile payments is going to damage PayPal is like being scared at the Halloween 16 movie sequel,” he said. “People have seen this before.”