I see no reason why we shouldn't replace the BP with a Firefly or Comet. from the wiki:
https://military.wikia.org/wiki/Black_Prince_(tank)
"By the time the Black Prince prototypes had appeared in May 1945, the Sherman Firefly had acquired a proven combat record, the Comet tank was in service and the introduction of Centurion was imminent. All these tanks carried the QF 17-pounder or a derivative; all had better mobility than the Black Prince and the Centurion had frontal armour of comparable effectiveness.[Note 1] The Black Prince had become redundant and the project was abandoned.[2]"
in game we could just give the firefly/comet extra frontal armor.
Or even better: Replace it with the M10 17pounder (also named Achilles (?)). This thing was present in Italy from 1944 onwards and I would really like to see it in a CoH game.
It could also get slightly better frontal armor than it deserved since there were M10 with 76mm guns that had add-on armor so it wouldn't be that much of a stretch to give M10 with 17 pounder some extra armor too. Also it was way faster than BP which could make up for its lower armor.