Well, one of my favorite "what ifs" is putting a Pegasus on the Skua
Yeah, might have worked... I can't specifically add any technical reasons why not, but get rid of the "fighter" expectation and call it what it is: a purpose built dive bomber.
And has been mentioned by others, Sticking a better Merlin into the Fulmar for a MK version should have been fast and easy.
I can't see why, to be frank. The intent was that the Fulmar wasn't supposed to have lasted as long in service as it did. It was an interim design until turret fighters entered service. Two things spoiled the pre-war Air Ministry plans for the FAA, the war and the admiralty getting the FAA back and attempting to "undo" the mess that had been made of the FAA in the 1930s. Note how the turret fighter spec disappeared and a single-seater spec appeared from the original replacements (from which eventually came the Firefly and Firebrand) for the specs that produced both the Fulmar and Roc?
Just seems like a waste when what the FAA really needed was a purpose designed single-seat interceptor with the capability and performance of the Hurricane or Spitfire, which is really what the admiralty (but not the Air Ministry) wanted, even before the Skua and Fulmar entered service.