I'd been imagining my first kiss would be something much more romantic.
Literal
First-kiss [topic-は], more romantic [one-なの] imagine was-doing [regret-のに].
The sentence-final のに 'even though / despite' here trails off to express regret or disappointment — the expected apodosis ('but it wasn't' / 'so I'm let down') is left implicit. なの after a na-adjective nominalizes the adjective into 'a (romantic) one.' 想像してた is the casual contraction of 想像していた 'I had been imagining' (い-dropping from ていた → てた is a routine spoken contraction).