Use AP Style title case in "Change Case > Title Case," leaving prepositions and conjunctions lowerccase
Sam Cantrell commented
Or even, if there was a dropdown menu under Title Case allowing us to select the particular ruleset we wished to use for the text frame.
So you would select your text frame, select Type > Change Case > Title Case > AP Style or Type > Change Case > Title Case > Chicago Style, etc.
Alternatively, you could add a document preference allowing you to set the particular ruleset for the document, but I'd think the first implementation would allow greater flexibility.
I love this idea, but would prefer an implementation that allows us to select the ruleset we want to use when using Title Case (Ex: AP style, Chicago style, etc), similar to how we can select the dictionary we want to use for spell checking.