My guess would be a speed coaster that focuses mostly on size and force or floater, as it's something they're lacking. Lightning Rod has speed, aggression, and ejector so that's covered. If they want to go something more edgy and focus more on an I305/DC Rivals style force machine, Mack is probably the way to go, and with Time Traveler going up at the sister park I could definitely see this being a possibility. A B&M hyper, on the other hand, would provide a nice, mild floaty ride but without inversions as a nice foil to Lightning Rod. And with this being the park's biggest investment, both are on the costlier side so I could see both being a good possibility.
Dollywood also doesn't have a spinner so I wouldn't rule out another high thrill spinner like Time Traveler, but while the two parks do share a lot of manufacturers and concepts, they don't usually have the same ride systems. Even RMC who was making like two ride systems at the time had to mechanically alter theirs for Lightning Rod. Herschend likes to go for firsts with their additions so it's pretty understandable why.
That being said, this could also be something we've never seen before.