One of the cardinal rules of videogame design is "Don't cross the genres."  And 99% of the time, split genre games fail miserably.

This is because players might like one of the genres but dislike the other, and instead of doubling your demographic, you turn off both sets of players.

However, this reasoning only applies to revolutionary breeds, which can be a large pill to swallow in one go.  But evolutionary convergence flips the rule on its head.

For instance, RPGs are considered a fairly hardcore market.  People who play fast-paced action, racing, or sports games are loath to try an RPG.  Too much strategy and not enough direct in-the-moment control, or too much story and too many cutscenes and not enough gameplay, or menu-driven systems feel like a Microsoft product; these are all common complaints from RPG detractors.  Such players would never play an RPG/Racing hybrid if they were die-hard racing fans with any one of these complaints.

Yet, slowly but surely, aspects of RPGs have snuck into many genres, including Racing games.  Nearly any game with an upgrade system can be traced back to RPGs, and owes D&D a thanks.

Early Racing games offered one car, and your skill was all that mattered.  Eventually, the player was given an option of cars, but the differences were purely visual, and the option was a necessity when Racing games became multiplayer.

At some point someone had the bright idea to create discrete statistics for different cars.  One might have a higher top speed with low acceleration and loose steering, and another might be just the opposite of that.  If cars in the game could crash, a stat was devised for how much damage the car could take before blowing up.

Now players could pick vehicles based on their play style.  This might have been an independent genius who created the stat system based on real car or racing stats, but I suspect he liked RPGs, too.

And if he didn't then the next guy in line most certainly did, for soon after that, upgrades were created.

At first, players might win a race and unlock a new car with better stats.  Then someone came up with the idea to let the player upgrade their original vehicle.  But to keep the player from upgrading all at once, the player would have to win money from races, then use the earnings to buy upgrades.

The final system is very similar to the RPG system of leveling up, combined with buying equipment, wrapped in a simplified package to meet the needs of the Racing genre.

Today, everything from Action bloodbaths like God of War and casual match-3 games have upgrade systems.

Similarly, RTSs were once considered a hardcore genre, but the casual Tower Defense genre has its roots in RTSs, combined with a dash of old arcade games like Space Invaders and Centipede.

Adventure games like MYST are too mind-bending and sluggishly dull for Action gamers, but navigation-based puzzles give Action games an intellectual depth that ultimately blends Action with Adventure.

Many genres have wormed their way into others, so much in some cases that we now use compound words to describe them, like Action-Adventure and Puzzle-RPG.  These genres were slowly introduced, so audiences could get used to them.

If someone said "I want to make an RTS-Racing hybrid," it would bomb.  But when a designer is playing an RTS and thinks "Hey, that mechanics might be cool in a Racing game," the then combo has a chance.  And if that's successful, perhaps another RTS mechanic gets put in the sequel, and so on until the line between genres is too fuzzy to detect.