I've mentioned some of these before, but I'll share again:
An organic, folk/Celtic song. Ed Sheeran has two songs on his new album (Galway Girl and Nancy Mulligan) that are both Irish-themed (though they are admittedly more pop than I'd like from Carrie if she were to actually attempt this sound) that I think sound awesome
On the same topic, I'd like her to continue experimenting with different genres/sounds, but maybe go even further and explore foreign/country-specific sounds (Asian, Spanish, African, etc.). This would almost necessitate a move into a poppier sound.
A prominent harmonica in a country song about either country living/big fields/the simple life or a longing relationship. Something that induces a nostalgic or sort of sad feeling. Maybe similar in mood to Oklahoma Wind, but with a more interesting melody.
Song (co)-written by Maren Morris
Duet (honestly, moreso Carrie as prominent backing vocals) with David Nail (like Let it Rain, Strangers on a Train, etc. or Eric Church's Kill a Word). I feel like she's duetted with most of the major acts within country music that there's not really anyone left that would result in a new or interesting duet that she hasn't already done.
A big ballad with an orchestra/strings. She doesn't have to release it if she doesn't want to be pigeon-holed, but I miss her big voice
Cool harmonies produced by Jay Joyce
An experimentally pop song by busbee, or Max Martin, Shellback, etc. from the pop world (closer to The Fighter than just country-pop like Relapse/Chaser).
Something new we haven't heard from Carrie vocally (e.g. high notes; see the previous point)
More generally, I really hope they put more effort into music videos this time around. They might view it as a place they can save money, but streaming is the medium nowadays and a good music video can really bring a song some attention (look at Somethin Bad). Most here would disagree, but for the most part I feel Carrie hasn't really had a music video that blew me away by either the production value (Cam managed it with Mayday) or the creativity of the story. I thought Blown Away and Something in the Water had so much potential and could have had much more epic music videos if they wanted to go that route.