According to Bozi Tatarevic, who wrote the Hagerty piece, "sources at Ford say we're looking at a new, higher-powered version of the turbocharged engine." Water cooler prognostication says Ford wants a better competitor for the Chevrolet Camaro 1LE, the Bowtie pony car getting 335 hp and 284 pound-feet of torque from a V6. Ford's four-cylinder trounces Chevy's four-cylinder, while the Chevy six-cylinder is 25 hp ahead but 66 lb-ft behind the Ford four, and the base V6 Camaro costs $1,105 more than the base Mustang.
Or... what if the new EcoBoost output is for the Mustang hybrid? Before Raj Nair moved to Multimatic, which builds the Ford GT, he was Ford's chief technical officer. In that role, he said in 2017 that the hybrid 'Stang would use an "EcoBoost-type engine," and have V8-beating torque "available with the electric motors." Motors. Plural.
The VIN decoder sheet fills in two V8 outputs and lists two as TBD. A known quantity is the 5.2-liter Voodoo V8 in the Shelby GT 350, rated at 526 hp. A 5.2-liter V8 listed as TBD we'll assume is the supercharged Predator V8 in the Shelby GT500. There's a 5.0-liter 4V V8 listed as 435 hp. That could be a typo; the Coyote made 435 hp until the 2018 model year, when it got bumped to 460 hp, and such errors are said to be "relatively common." The remaining 5.0-liter V8 could be for the Bullitt, which currently makes 480 hp.
Check out the Hagerty piece to see the VIN document. When asked about the paperwork, Ford would only officially say it "is always elevating Mustang and will share more exciting news this spring."