Contrary to what you might have heard or believe, investing in the stock market isn't hard. Rather, the difficult aspect of investing has to do with being patient and letting your investment thesis play out over time. Allowing your initial investment to compound many times over is what can turn a nice unrealized gain into a life-altering amount of money.

Over the next quarter of a century, three special stocks have the opportunity to generate mammoth returns for their investors. I'm talking about turning a $25,000 investment into $1 million or more -- representing a 3,900% return -- in 25 years or less.

A messy pile of one hundred dollar bills.

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Teladoc Health

Although small-and-mid-cap stocks would seemingly have a better chance of a 40-fold increase in market value over the next quarter of a century, the one special stock that truly stands out as a game-changer is telemedicine giant Teladoc Health (NYSE:TDOC). If my prediction proves accurate, Teladoc would near a $1 trillion valuation, up from its current market cap of $23.9 billion.

In recent months, concerns have mounted that Teladoc's growth would slow. After all, it was in the right place at the right time during the coronavirus pandemic. As infections soared in 2020, Teladoc's platform handled 10.59 million virtual visits, up from just 4.14 million in the previous year. Presumably, virtual-visit growth will tail off a bit as the U.S. economy continues to reopen.

However, this short-term narrative overlooks very clear long-term shifts and growth trends that are already in place. For instance, telehealth services are significantly more convenient for patients, and they can provide physicians with easier access to their chronically ill patients, who may require more oversight. This simplification and personalization of the treatment process is liable to result in improved patient outcomes, which health insurers are bound to appreciate since it'll mean less money out of their pockets.

Further, Teladoc's sales grew by annual average of 74% in the six years leading up to the pandemic. While 74% annualized growth is clearly not sustainable for the next quarter of a century, it demonstrates that Teladoc's platform had plenty of momentum long before the pandemic hit.

To differentiate itself even further, Teladoc acquired leading applied health signals company Livongo Health in the fourth quarter of 2020. Livongo collects copious amounts of data on people with chronic illnesses, and with the help of artificial intelligence sends tips to its subscribers to help them lead healthier lives. Before being acquired by Teladoc, Livongo was already profitable on a recurring basis and practically doubling its sales annually.

As of the midway point of 2021, Livongo had 715,000 enrolled members, with a focus on diabetes.  For some context here, we're only talking about a 2% share of the U.S. diabetes market (an estimated 34.2 million people).  With Livongo adding hypertension and weight management to its platform, the prospective pool of patients that could benefit from its services in the U.S. is enormous.

Teladoc is a personalized care behemoth in the making, and I suspect it'll make a lot of long-term investors rich.

A smiling mother leaning out the window of a parked car, with her children in the back seat.

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Root

Another special stock that could completely shake up an industry that's just begging for innovation is insurance company Root (NASDAQ:ROOT). Although Root comes with a higher degree of risk and near-term volatility than the other companies on this list, its innovation could power it to a market cap north of $70 billion in a quarter of a century, or less.

For decades, the auto insurance industry has priced its policies based on metrics that, frankly, tell the insurance company nothing about your driving habits. Everything from a person's credit score to their marital status has been used in some way to price out auto insurance policies. What Root brings to the table is a brand-new way of pricing insurance policies up front that'll actually take into account metrics that matter.

Root's secret sauce is its use of telematics. Utilizing highly sensitive instrumentation found in smartphones, such as the accelerometer, magnetometer, and gyroscope, Root can gather mountains of information on driving habits, G-forces while braking, accelerating, and turning, and other pertinent factors that really do paint a picture of how safe or dangerous someone is on the road.

The big question is, can this data be used to effectively price policies? Even though Root is a very new company with a unique idea, its early loss ratios have been promising. Between the first quarter of 2019 and Q1 2020, its direct accident period loss ratio declined from 106% to 80%. Anything below 100% is considered a profitably written policy. Though this decline could be partially attributed to people driving less during the pandemic, what stands out is that it fell, yet again, in Q1 2021 to 77%. In fact, the company recorded a nominal gross profit of $6 million during the March-ended quarter. 

What's more, Root plans to push into new verticals beyond auto insurance, such as renter's insurance. The potential to retain members and make quick add-on sales is simply too alluring.

Despite the expectation that it'll lose quite a bit of money via marketing and expanding its reach over the next couple of years, the company's early operating data implies that Root is onto something that could really shake up the insurance industry. As long as access to financing remains abunant during its very early growth phase, we may well be looking at the most-successful insurance stock of the next 25 years.

A veterinarian holding up a small dog.

Image source: Getty Images.

Trupanion

The third special stock that has the potential to turn a $25,000 investment into $1 million or more over the next quarter of a century is specialty insurance company Trupanion (NASDAQ:TRUP). Yes, another insurer. But whereas Root is shaking things up for humans, Trupanion aims to change the game for companion animals, such as cats and dogs.

If you're wondering "Why Trupanion?" the answer can be found by taking a closer look at pet expenditures and ownership statistics. For example, 56% of households surveyed by the American Pet Products Association (APPA) owned a pet in 1988. By the 2019-2020 survey from APPA, this figure was up to 67%, or nearly 85 million households. It's been at least a quarter of century since year-over-year spending on companion animals has declined in the U.S., and nearly $110 billion is projected to be spent on our furry family members this year alone. In other words, the pet industry is about as recession-resistant as they come. 

Of the roughly $110 billion in estimated pet sales in 2021, $32.3 billion is forecast to go to veterinary care and product sales. Trupanion aims to help pet owners manage these potentially unexpected expenses through lifelong health insurance plans for their companion animals.

As of the end of March, Trupanion was approaching 944,000 enrolled pets, a majority of which are part of its subscription segment.  While this might sound like a lot, it actually only represents about 1% of the addressable market in the United States. In the U.K., for instance, about 1 in 4 pets have health insurance. If Trupanion were to reach a 25% penetration rate in the U.S., its addressable market would skyrocket to almost $33 billion. For context, Trupanion is expected to bring in around $680 million in full-year sales this year, according to Wall Street.

If you're worried about competition, you shouldn't be. For the past two decades, Trupanion has been building invaluable rapport with veterinarians and staff at the clinical level. After all, veterinary clinics are effectively the ambassadors of Trupanion's service. Further, Trupanion is the only major companion animal health insurer that offers software capable of handling payment at the time of checkout. This means less hassle for its subscribers and a positive experience for veterinarians and their staff.

Ultimately, Trupanion is still in the very early innings of its growth. As North American market penetration increases, we could really see its profit potential soar.

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.