definitely interesting mood. This could become a great scene if you want to continue working on it.
For starters, I'd give the exposure one more bump up.
As for grasses: your surface works quite well, but if you want grass models, populate them with the use of a distance shader, until the individual grass cannot be distinguished any more. Then blend into "just surface". The tricky thing is to match moloring so you won't notice that the grasses are not models anymore.
Frank