What will the stock market do in 2021? There's no way to be sure. However, there are several reasons to be optimistic. COVID-19 vaccines could help bring a long-awaited end to the pandemic. Significant progress toward this goal would likely boost the economy and the market. Historically, the stock market tends to perform well during the first year of a new president.

This could mean that investors will enjoy another year of strong returns as Joe Biden moves into the White House. But which stocks are poised to be really big winners? Here are three stocks that'll likely make you richer in a potential Biden bull market.

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1. Brookfield Renewable

Renewable energy could receive a boost next year in a couple of ways. A rebounding economy would drive higher demand for energy, in general. Also, the incoming Biden administration plans to prioritize carbon reduction -- which certainly would make renewable energy sources even more attractive. As a leader in renewable energy, Brookfield Renewable (NYSE:BEP) (NYSE:BEPC) should be a prime beneficiary.

Brookfield Renewable operates hydroelectric, wind, solar, and electric storage facilities across four continents. It currently has over 19 gigawatts of energy production capacity, up from three gigawatts five years ago. 

The company should deliver significant growth over the coming years. Brookfield Renewable's development pipeline includes 18 gigawatts of capacity. It's rapidly scaling up its solar power capabilities, in particular. That appears to be a smart move, considering that solar and wind are now the cheapest sources of bulk energy production. 

Just how much can Brookfield Renewable really grow? The company projects it will achieve average annual total returns of close to 15% over the long term. That means an investment in the stock today could double within five years if Brookfield Renewable hits its target. 

2. Innovative Industrial Properties

Let's move from one green revolution to another. Legalized cannabis has more momentum in the U.S. than ever before. On Election Day, voters in five states supported the legalization of recreational and/or medical marijuana. Innovative Industrial Properties (NYSE:IIPR) is set to profit from the rapidly expanding U.S. cannabis market.

Many U.S. cannabis companies can't list their shares on major stock exchanges. They also face challenges accessing financial services that are readily available to other companies. Why? Federal laws about cannabis stand in their way. Neither of these is a problem for Innovative Industrial Properties, though. It's a real estate investment trust (REIT) that owns and leases out medical cannabis properties, but its business doesn't violate any federal laws.

These problems for cannabis operators present big opportunities for IIP. It buys properties from medical cannabis companies then leases the properties back to them. The cannabis operators get much-needed cash, while IIP gets a steady revenue stream that it can use to invest in even more properties.

IIP stock has more than doubled so far this year as the company increased its number of properties from 46 at the end of 2019 to 64 as of November. But IIP still only does business in 16 states -- less than half the total number of states that have legalized medical cannabis. Many of the medical cannabis markets in the states where it already operates remain in their early stages. I think this cannabis-focused REIT can and will deliver tremendous returns over the coming years, just as it's done in recent years.

3. Square

Small retailers have been hit especially hard by the COVID-19 pandemic. You might think that a company like Square (NYSE:SQ), which provides payment processing and other solutions to small- to medium-sized businesses, might have also been slammed. Sure, Square's business was negatively impacted to some extent. However, the company reported fantastic Q3 results. Its stock has skyrocketed nearly 250% year to date.

The main reason behind Square's success is its popular peer-to-peer payments app Cash App. The number of active Cash App users nearly doubled year over year in Q3. Some might wonder if this momentum could fizzle as COVID-19 worries subside. I don't think so. My view is that the use of digital payment alternatives like Cash App will increase in 2021 and beyond.

Remember, too, that Square's payment processing for retailers should gain a lot of steam if the economy roars back beginning next year. Square's new features, including QR code functionality for contactless in-store transactions, make its platform stickier than ever for these customers.

Arguably the strongest reason to invest in Square is that it offers an entire ecosystem of products and services to businesses. Look for the company to expand its ecosystem for individuals as well. Square's already moving in that direction with its plan to acquire Credit Karma's tax preparation business. If a Biden bull market arrives next year (and perhaps even if it doesn't), Square should make investors significantly richer.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.