Slitherine

Would this be a possible change, perhaps as an optional rule, for GS 2.0?

In the game as it stands, Yugoslavia is a neutral unless invaded by the Axis (or Allies). In reality, Yugoslavia actually joined the Axis on 25 March 1941. But then on 3 April a coup changed the government in Belgrade and on 6 April Hitler invaded.

Could the game be modded so that on Turn 29 there is a 50% chance that Yugoslavia joins the Axis, and if not a 50% chance on Turn 30 that it joins the Allies? This would be "realistic," and if there was a coup on Turn 29 the Germans would have to strike on the Axis Turn 30 (6 April) to get the surprise bonus before (50% chance) Yugoslavia joined the Allies?

I know some players will not like such a possible optcome. Optimally could such a feature be added as an optional choice to be checked off on the menu with the other options?

I more or less agree with this idea.

And the Yugo units should have strength 1 or 2. Armed resistance to the Axis essentially non-existent, with most Axis casualties resulting from traffic accidents.

I don't think such a rule would be very "realistic", especially it is doubtful that Yugoslavia would have joined the Allies (which in fact would be only the British Empire in 1941). As far as I know the new Yugoslavian government did not make any declaration of loyalty and it would be likely that Yugoslavia would have stayed neutral.

Look what's being written in Wiki:
Prince Paul submitted to the fascist pressure and signed the Tripartite Treaty in Vienna on 25 March 1941, hoping to still keep Yugoslavia out of the war. But this was at the expense of popular support for Paul's regency. Senior military officers were also opposed to the treaty and launched a coup d'état when the king returned on 27 March. Army General Dušan Simović seized power, arrested the Vienna delegation, exiled Paul, and ended the regency, giving 17-year-old King Peter full powers. Hitler then decided to attack Yugoslavia on 6 April 1941, followed immediately by an invasion of Greece where Mussolini had previously been repelled

On March 25, 1941 in Vienna, Dragiša Cvetković, Prime Minister of the Kingdom of Yugoslavia, signed the Tripartite Pact. On March 27, the regime was overthrown by a military coup d'état with British support, and the 17 year old King Peter II of Yugoslavia seized power. General Dušan Simović became Peter's Prime Minister and the Kingdom of Yugoslavia initially tried to dissolve the Pact but later declared adherence to it.

The initial agreement of the document also regarded Yugoslavia's acceptance of the free movement of German troops around the country; this was unsatisfactory to the Führer, and resulted in the Invasion of Yugoslavia.

Postponing Operation Barbarossa, the Germans simultaneously attacked Yugoslavia and Greece. Starting on April 6, the German Air Force (Luftwaffe) bombed Belgrade for three days and nights. German ground troops (Wehrmacht Heer) moved in, and Yugoslavia capitulated on April 17.

So Yugoslavia was pressured to sign the tripartite pact hoping to keep Yugoslavia out of the war. The pact allowed Germany to move troops through Yugoslavia, but it didn't mean that Yugoslavia would allow their forces to be used freely by the Germans. The production would not go to Germany either.

This is similar to Sweden allowing for Germany to rail troops to/from Finland while remaining neutral.

This pact was only active for 2 days before a coup overturned it. Germany invaded Yugoslavia after the coup because the government had changed from pro-Axis to pro-Allies.

I don't think Yugoslavia would have joined either side if Germany hadn't invaded. There are many other minor powers that were either strict neutral, pro-Axis or pro-Allied. In GS they're usually considered to be out of the war. Sweden is sending their iron ore to Germany (being pro-Axis). Spain can if the French Armistice offer is rejected send tungsten to Germany. They can even join the Axis if north Africa becomes Axis controlled. Turkey can also join one of the sides.

Since GS v2.0 lacks a diplomacy screen I think it's best to let neutral powers be neutral and let events on the battlefield decide what happens to their status.

That sounds about right... but I'd still recommend that Yugo's forces be radically downsized, along the lines of what was done for Denmark/Sweden. Axis casualties in Yugo were trivial.

Most of the time I take Yugoslavia in one turn anyway, so what is the point? Any reasonable Axis player simply waits until Romania enters then places three mechanized/armor there along with a couple of Tacs and Voila! Belgrade generally falls immediately.