Given the 1939-1940 time frame, there is probably not a whole lot that could have been done to drastically improve the Hurricane until the BoB was basically over. Actually, it was a fine fighter for the time it was most needed, and an RAF consisting solely of Huricanes could still probably have defeated the Luftwaffe in the BoB. I think introduction of a bubble or blown canopy would be a cosmetic improvement at best. Introducing a newer thinner wing would probably require a virtual redesign (plus the new wing would be less easily adapted for ground attack). Probably, as others have noted, the best improvement would be replacing the eight .303 MGs with a more potent mix of .50 cal guns and 20 mm cannon, but the Spit had the same "flaw".
I think the best thing that could be done with the Hurri is what happened. Convert it a dedicated ground atack machine as soon as feasible, but unlike the Stuka and Sturmovik, it would be a ground attacker that retained at least some abililty to mix it up with fighters if necessary.