Q. When the word “coke” is used as a generic term to refer to any kind of soda, should it be capitalized?
A. The word “Coke” is a trademark, and it’s a noun—so it doesn’t fit the pattern of a wildly successful trademarked name that acquires a generic (lowercase) sense as a verb in common usage (e.g., Google > googled). It’s best therefore either to capitalize it or to use a generic word like “soda” or “cola,” depending on which is meant (apparently, it can vary by region).
If you’re writing or editing a novel, you might consider another alternative. For example, you could use context to let the reader know that the speaker doesn’t literally mean Coca-Cola:
“I’ll have a Coke,” he said. They served only Pepsi, but he wasn’t the type to make such distinctions.
Or, yes, you could lowercase the term and hope readers will understand that a lowercase c means the term is being used generically. But unless your style is to lowercase things that are normally capped, “Coke” with a capital C is still probably the best option. For more on this issue, see “Can I Put an iPhone in My Novel?” at CMOS Shop Talk.