Is the most likely prediction at the moment Catfish/Ezra/Stormzy then? If it is, that is a huge risk on TRNSMT's end.
Stormzy will be going in off the back of headlining Glasto but on the other hand does have a very questionable popularity with the typical TRNSMT audience, especially given the other two headliners.
George Ezra, to be fair, is headlining a lot of festivals of a similar size (Isle of Wight, Parklife, Latitude), so bookers obviously do have some faith in him. He is undoubtedly one of the hottest acts in the UK right now but I can see the issue with him doing festivals. A lot of the people attending his concerts are likely to be children and their parents, and generally people who wouldn't tend to go to a festival. I'd be interested to see how he'll do as a headliner (I don't think Neighbourhood is a fair example to use, there was obviously a lot of hype around the first year and he is almost definitely a better ticket seller than Richard Ashcroft. This year's undercard is arguably weaker too so that wouldn't help with ticket sales).
Catfish have been headlining festivals slightly smaller than this for a couple of years so, with the new album, it really wouldn't be a huge push to see them headline. The issue is Catfish shouldn't be the 'safest' headliner on their first ever major festival headline spot. If they were there with two established headliners, they wouldn't be a problem at all but now they seem slightly underwhelming.