I think Ray is probably the most likely, even if he has yet to fulfill any of his promise. Simply because if nothing else, he's probably the second best CF defender on the 40-man, so in case of injuries he'd be up. I would expect Ethan Small to get the Burnes-like introduction, a mid-late season callup to the 'pen with the intention to start in the future. It's easy to think that because he was in A-ball in 2019 he was far away; but I would think the way he was deployed (Short starts with time inbetween) was more due to having already pitched a lot of college innings, and that he was in Arizona and then Appleton mostly for coaching and working on things. He was a very advanced college pitcher, and I would've expected him to have pitched in AA (Maybe a very brief stint in A+) in 2020 had there been a season. File and Bettinger I think may be more injury insurance this year so circumstances would dictatea their chances.

But it's hard to know who took a step forward during 2020 and who didn't. In a normal year someone who had a breakout might have ended 2019 in A+, but gotten an early-season promotion (or even started) in AA in 2020, and ended the year in AAA after yet another promotion. That kind of player would be likely to be in line for a callup. Whereas if said player stayed in A+ most of the year in 2020, even if he did well, that would suggest a 2021 debut is unlikely. But we as fans don't really know who might be among the former and who might have been among the latter during summer camp and/or instructionals.

But you never know with pitchers especially. Don't know if we really expected Bickford to debut in 2020 despite his impressive 2019. Many pitchers in recent years who have gotten relief debuts after having not pitched above A+ or so; in 2020 much of it was due to necessity, but guys like Jordan Hicks stick out from earlier years. So I expect some unexpected reliver to debut. Someone out of Ashby, QTC, Andrews, Sanchez, Barker; whoever took the greatest leap forward.

So to put out some actual predictions to be mocked in the future:
Corey Ray will be the first MLB debut of 2021 relatively early on, followed by Ethan Small shortly before the all-star break after impressing in the minors. Dylan File will make his debut in an August doubleheader as the 27th man. Aaron Ashby gets a September callup and pitches 3.1 innings out of the pen.

And to name a dark horse: Brice Turang. It would take multiple SS injuries, but I think someone with that combination of defense, baserunning and plate discipline is probably already a replacement level player. I think the Brewers will want to wait until he's more than that before they start his clock, but clubs do promote players like that early out of desperation sometimes. See for example Nico Hoerner for the Cubs. Again I don't actually expect it, but if I had to pick an unlikely but still realistic name it'd be him. Probably requires all of Urias, Arcia and Mathias to be injured at the same time after the trade deadline has passed though.