Deserved A Sequel
The Dam Busters is one film that definitely deserved to have a sequel produced; preferably, with the same production team and director.
The plot would be simple. It could have focused on what 617 Squadron did in the time from after the Dams' Raid until V-E Day. That story almost makes the original raid pale in comparison.
It could have had such things as the disastrous attempt at the Dortmund-Ems Canal, which reduced the squadron to a mere eight crews with Micky Martin as acting CO. Afterwards, they trained to become experts at high-level precision bombing and Leonard Cheshire took over as CO. Under Cheshire they perfected marking of targets and became the most accurate squadron in Bomber Command. The following spring, they got the 12,000 pound "Tallboy" earthquake bomb that Barnes Wallis developed. (The success of his Dams' bomb gave him the credibility to proceed with ideas he had had since 1940 but had been written off as "crackpot".) With the Tallboys they were able to destroy V1, V2 and V3 sites in the Pas-de-Calais which were buried under tons of concrete. Next, under new CO Willie Tait they sunk the battleship Tirpitz and began a campaign against concrete U-Boat pens. Finally, in the last months of the war, under RCAF officer Johnny Fauquier, they got Wallis 10-ton "Grand Slam" bomb that they used to smash the sub pens in Hamburg and effectively isolate German pockets of resistance by eliminating massive bridges.
So, this is one of the rare historical films that could have actually benefitted from a sequel.