Yep, it's nowhere near an exact science.
Making a cinematic film (as opposed to extravagantly budgeted TV level plot) involves more than just the story though that remains an important part. Certainly INS is probably one of my least favourite films (although a large problem for me with it is the lack of an actual debatable problem in the central plank of the story, but it's true I couldn't care less about the two other races involved. I do attribute this to the writing but you also have to give the actors some responsibility as well. A good actor can make something out of duff dialogue - Montalban and Stewart to name a couple - and some can't) but going back to the TV shows you have episodes such as 'City on The Edge of Forever', 'Journey to Babel' and even broader eps like 'Balance of Terror', 'The Doomsday Machine' which are more in their scope the sort of stories that could be adapted and made cinematic in the right hands.
This is not to suggest O&K are necessarily those hands, but on an adaptable level they could serve as the basis for something more epic and cinematic and they still retain a cross section of Trek-ness.
'If the Apocalypse starts, beep me!' - Buffy Summers
'The sky's the limit.....' Jean-Luc Picard, 'All Good Things'
courtesy of Saquist