Well,
there's only one thing I badly want to see added to this game, but I doubt it ever will and it would not see much use in a 1940 dlc anyway (unless the suggested 1940 Italian missions get added and you could start a new core that can show up again in Africa and/or the Eastern front in later AO DLCs), so I will pick something different and smaller (that's not likely to get added either, but I think it would be fun and something new nevertheless).
It's not a very well known fact, but after the Dutch laid down their weapons on the 14th of May and officially surrendered one day later, an exception was made for the southwestern province of Zeeland. I believe the main reason for this was to allow the French forces that had arrived in southwestern Netherlands a chance to retreat. So the Dutch and French actually fought the Germans together there from the 14th until the 17th of May and then surrendered after the Germans largely destroyed the historical center of the provincial capital of Middelburg by artillery bombardment, in a way a small repeat of the bombing of Rotterdam two days earlier.
So I would nominate the Battle of Zeeland as a nice addition, since it would be something new, is a nice opportunity to add some French troops to support the Dutch and I also think the map is quite interesting too:
Apart from just conquering the map, there's room for nice bonus objectives too, for instance, intercepting retreating French forces that basically followed the lower of the two red lines of the German advance and then tried to cross the Westerschelde water at Vlissingen to Breskens. The fat green lines show the locations of the main allied defensive lines at Bathstelling (Bath line), Zanddijkstelling (Zanddijk line) and Sloedam, at the time there was only a small dike or dam between the islands of Walcheren and Zuid-Beveland (one important reason why current maps wouldn't work as well for WW2 scenarios, there's now a broad area of land reclaimed from the sea there), basically a killing zone which Canadian soldiers found out the hard way when they had to cross there 4,5 years later.
It's a perfect example of a small unknown conflict within the larger conflict of WW2. Take the tragic/heroic tale of French general
Marcel Deslaurens, for example. To quote the Wikipedia page that I linked:
On Walcheren he also took command of men from the 68th Infantry Division. His main concern was to defend their position and prepare an organized escape back to France, and to raise the moral of the fatigued troops. On 17 May their positions were attacked by German artillery and airplanes, with Deslaurens inspecting his troops without regard for danger. Under severe pressure, the troops retreated to Vlissingen; orders were given to embark on French warships (under the command of admiral Charles Platon) that evening. Amid the chaos of French troops that continued to pour in, and given the demoralized state of those troops, Deslaurens himself grabbed a rifle and covered the retreat, setting up a position with whatever soldiers and officers he could detain, some four hundred meters from the embarkation. By 22:15 all had been evacuated except for Deslaurens and his small defensive group, who were all killed.
He was one of thirteen French generals who died in May/June 1940. Originally buried on the Noorderbegraafplaats in Vlissingen, his remains were later transferred to France.
When I read this small piece of info, I could imagine an entire movie being made about this event alone. And speaking of movies, next year will see the release of the most expensive Dutch movie until now (not sure how big a thing that is

):
The Forgotten Battle (trailer for some reason not on youtube yet but
here on twitter). It is set in the same province of Zeeland in 1944 when Canadian, Polish and British troops had to liberate it to give the Allies access to the harbour of Antwerp, after the Market Garden fiasco. The
entire operation turned into a series of bloody battles after the Germans had plenty of time to reinforce the area because Montgomery decided to go for the bridges over the rivers near Nijmegen and Arnhem instead of securing a vastly improved supply situation which access to Antwerp would have given. This battle of the Scheldt might** have been the biggest (and bloodiest) Allied operation on the Western Front in 1944/1945 after Overlord, Battle of the Bulge and Market Garden, but not many people have heard of it. So hey, the Battle of Zeeland may not make the 1940 DLC, but there's still plenty of time to implement the same map for the Battle of the Scheldt in a 1944 Western front DLC!
I was born and lived in Goes from age 0-12, lived in Vlissingen from age 12-18 and my parents are still living in Middelburg today. All of these places are on the map I posted. So I think you can guess why I know about these WW2 events while some of them are still called
The Forgotten Battle 
Really don't care about having an in game hero named after me, but at least this all served as inspiration for this post, so if someone enjoyed reading this it was worth typing this. Btw, I never even knew about General Deslaurens until today, so at the very least this "Kerensky contest" brought this 'little' story to light. The general finally received a monument in Vlissingen in 2001 apparently, 4 years after I moved out of town and the province myself. So because of this lil' contest/idea/whatever I managed to dig up this story today about 1 of 13 French generals that died in 1940 during the German invasion in May/June 1940. A French general dying in the Netherlands in 1940, all but forgotten...makes me once again think about how many young kids have been forgotten.
** At least I (for whatever that's worth) can't think of any other.