Regularly buying shares of growing companies is an easy way to build wealth in the stock market. But to land those elusive multibagger investments, you want to look for companies that still have a large market to expand into.

To give you some ideas, here are two stocks that can deliver monster returns over the next 20 years.

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1. Roblox

Roblox (RBLX -2.97%) stock has surged 136% year to date. The gaming platform is attracting users in droves, as its user-created game development strategy is proving effective at releasing on-trend experiences that get players engaged. As the company continues to explore new monetization strategies and leans into artificial intelligence (AI), the stock could deliver spectacular returns.

Roblox now has more than 111 million daily active users, and these users are spending over 27 billion hours on the platform. It has been effective at monetizing its user base with engaging experiences. Adjusted revenue, or bookings, reached $1.4 billion last quarter, up 51% year over year. This revenue primarily comes from players buying virtual currency to buy cosmetic items for their digital avatars, like clothing and other premium features.

Management is also working on growing advertising revenue, particularly with things like video ads, where players can be rewarded with virtual currency for watching an ad. Advertising could become a big business for Roblox, considering that its user base is skewing older and less dependent on children under 13 where laws would restrict advertising opportunities. In the second quarter, 64% of its daily users were 13 and older, and this percentage has been trending higher in recent years.

Initiatives like video ads and integration of AI features in the content creation process have the potential to create a lucrative flywheel of growth for the company. AI can make it easier to make content, leading to more frequent content releases, and more users spending time on the platform and engaging with ads.

Roblox is on track to become the largest gaming company in the world by revenue. Management believes it can reach 1 billion users, which would lead to substantial opportunities to grow revenue in a gaming industry valued around $200 billion and growing.

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2. MercadoLibre

If you're looking for a compounding growth machine that could be undervalued and absolutely crush the market's average return over the next five years, look no further than MercadoLibre (MELI -6.77%).

MercadoLibre is the leading e-commerce and fintech business in Latin America. It operates an online marketplace, credit services, mobile payments, and logistics network to lock in millions of customers to a strong ecosystem of services.

It's been growing at consistent, high rates for many years. Despite its two-decade history, the business reported a remarkable year-over-year increase in revenue of 53% on a constant-currency basis last quarter. This strong growth signals a massive addressable market in Latin America, which has a large population lacking access to basic financial services.

MercadoLibre has created great synergies between its online marketplace and fintech services. Gross merchandise volume, or the total value of transactions on its marketplace, topped $15 billion last quarter, rising 37% year over year on a constant-currency basis. It has more than 67 million fintech users, driving its total payment volume to $64 billion in Q2.

These services provide multiple revenue streams from transaction fees and other services, including its fast-growing credit card offering, that are boosting profits. It reported a healthy operating profit margin of 12% in Q2, and this margin should continue to trend higher.

It's also great to see MercadoLibre using its scale to deliver more value to consumers and widen its competitive lead. It has reported higher margins over the last year while expanding free shipping offers to customers. It is also expanding into new businesses that expand its addressable market, such as advertising, which grew 38% year over year last quarter. Its new credit card product is also growing rapidly, with the credit portfolio up 91% over the year-ago quarter.

On a price-to-sales basis, the stock is cheaper today than it was five years ago. In 2020, MercadoLibre traded at more than 15 times sales, and it now trades at a multiple of 5.1. This lower valuation is a great opportunity to buy shares before Wall Street realizes it has greatly underestimated MercadoLibre's growth prospects.