。
I don't really think that's a good idea.
Literal
Too-much good idea is-not [quotative-と] think-[polite-ます]-but.
A classic case of sentence-final が as a softener. Nothing actually follows the が — it's left hanging to soften the dissent, inviting the listener to fill in 'but... [maybe consider?]'. This trailing-が is one of the most common polite-disagreement strategies in everyday Japanese, far softer than a blunt ~と思います.