Here are some options I could get excited about. On this list, I have 3 categories: Opener (15-20 mins), Special Guest (30-40 mins), and Co-Headliner. Obviously if Carrie only has one act out on tour with her, I hope it comes from the Special Guest category (aka strong sales, radio, and popularity). If she has two, I'd like one from Opener and one from Special Guest. Also, within each category, I ranked most preferred to least preferred - top to bottom. Overall, I'd be pleased with any of these.
Opening Act (15-20 mins):
MADDIE & TAE (Had pretty great debut sales for their album)
KELSEA BALLERINI
JENNIFER NETTLES
DAN + SHAY (The shorter one can hit come impressive notes that would be fun to see with Carrie)
BRANDY CLARK
MICKEY GUYTON
A THOUSAND HORSES
Special Guest (30-40 mins):
SAM HUNT (can't deny his popularity - and I like his music. Sure, not country minus some themes and a banjo or two, but quality in my opinion)
LITTLE BIG TOWN (I'm picturing a 'Jolene' --> 'Girl Crush' --> 'Two Black Cadillacs' mashup - think about it...)
KACEY MUSGRAVES
DARIUS RUCKER (he bores me to tears, but he's got credibility)
KIP MOORE
Co-Headliner (same set length as Carrie):
ZAC BROWN BAND
(this is obviously a LONG shot, but could be the reason we start to see Carrie (and ZBB for that matter) in Stadium venues. I'd shoot for ZBB performing first ('opening' for Carrie with the same length of setlist) for 2/3 of the tour. Let ZBB close for the hipster towns (Seattle, Portland, Austin, Denver, etc) and Carrie close for the rest.