Okay, maybe it’s a decent idea. But I’m not sure the customer would want something that they’d have to make choices on. That’s why there’s a large choice in Sodas right now: You want what you want, not necessarily a choice once you have it.
Besides, I get the feeling the taste of the included flavors would suffer. Think of it: you get your choice of four different flavors, instead of a good flavor you get sixteen mediocre flavor possibilities. The formulation for a straight Cola soda will be different from lime-flavored cola flavor will be different from cherry-flavored cola, while something made to cross-flavor will end up either uninspired or unbalanced.
Not only that, but imagine if you could pop the caffeine button separately. I could see someone saving eight or ten of these, then popping the caffeine buttons and pouring the liquid into another beverage for a super-caffeine buzz.
Like I said, a decent idea. Now work on it and make it better. Or better yet, let it be — usually we KNOW what we want.