Just this. Every land-based strike aircraft from Midway, combined with every torpedo plane, dive bomber, and (assigned) escort fighter from TF 16 & 17, had arrived over the Kido Butai all at once? Feel free to pick a time. I'd say you have to cancel Fletcher's useless aerial scout missions to the north, and have all of the three US carriers with fully working elevators. Also, I'd have to rule that the critical hit knocking out the fuel pumps at Midway misses.
Opinions? Ideas for scenarios?