RIght now I’m enjoying the hell out of a bag of Jalepeno Cheetos- to me, they’re way better than regular Cheetos and could even make them obsolete (on second thought, probably not). But it got me thinking- exlcuding Diet Coke, Bud Light and others items popular due to less calories, etc.- what would be the most popular/successful offshoot of an exisitng product?
No cite, but I’d guess crunchy peanut butter came first, and creamy came later.
Sugar frosted corn flakes from plain corn flakes.
The myriad candies that came from a plain chocolate bar.
Bagels with other stuff added in. Hardly anyone orders a plain bagel these days.
I vote for Honey-Nut Cheerios.
Chocolate milk
I wasn’t clear int he OP, sorry, this is more what I had in mind- a specific brand name off-shoot of a specific brand name prodcut.
Honey Nut Cheerios is the same thing as your OP…it’s the honey-and-nut-flavored Cheerios like your puffs are the jalapeno-flavored Cheetos…??
I think Frosted Mini Wheats are a 400% improvement over shredded wheat.
Cool Ranch Doritos?
New and Improved <insert product name>. 
I don’t know if this is most successful, but I LOVE those Mint M&Ms that come out around Christmastime.
Also, Coke Zero (and to a lesser extent, Sprite Zero) is completely boss.
I’m quite partial to the vanilla Coke and the lime Diet Coke.
Purina One catfood’s basic flavor is poultry based, but my cats’ vote goes to salmon & tuna flavor.
The iPhone from the iPod?
Would you consider Coco-Pops a Rice Krispies offshoot? They’d get my vote - "
Just like a chocolate milkshake, only crunchy!"
Mmmmm, Jalapeno Cheetos are delicious. They’re like a corrective experience from Flaming Hot Cheetos.
Jalapeno Cheetos = cheetos + hot + flavor
Flaming Hot Cheetos = cheetos + hot - flavor + sour
Cherry Coke. And Nestle crunch crisp. I have a very sweet tooth.
Just about any ice cream flavor, beyond vanilla, chocolate and strawberry.
Half-and-half.
Bush’s baked bean varieties.
Jam/Jelly donuts?
That’s Cocoa Krispies. Coco-Pops are a Kix offshoot.
What about Chocolate Chip Cookie Dough ice cream? Originated by Ben & Jerry’s, it got to the point where people would drive to their main store to get it when it first came out.
Maybe I wasn’t clear again, Honey-Nut Cherrios is what I meant exactly, unlike the other early posts- generic chocolate milk, flavored bagels, etc. 
I haven’t met anyone who doesn’t agree that Olive Oil and Black Pepper Triscuits justify the decades of simple Triscuits’ existence.
You just met me. TOO MUCH BLACK PEPPER! Yow!