In 1918, Antony Fokker brought a new fighter, the V-11, to a fighter competition. Von Richthofen flew it and said it was the worst airplane ever. Fokker took the airplane back for a weekend and added one bay to the fuselage, and a bit of vertical fin. Von Richthofen said it was the best fighter ever. They called it the D VII.
The Me-210 could have been fixed over a weekend, but it took much longer to recognize and remedy in much the same way as the Fokker.
It wasn't used in CAS/AT, so judgments on it's performance in the role may be presumptuous. It was effective at flying over England and not doing much. It was an effective bomber destroyer in the absence of escort. There actually was a variant planned to take the BMW 801 engine.
Had the Me-210 had that weekend in Fokker's shed, early in it's life, it might have been a contender, for a while.