it'd look like Conflict of Nations

They already made this game... its called Conflict of Nations

Then this game will have all the modern weapons.

A mess... however it would be interesting reading the battle reports of drone attacks and chemical warfare. Cyber attacks and endless trade sanctions. Indeed much more would need to be added to the game.

larger nations would roll over smaller nations and few coalitions would be formed.

As I said in many post, this is not our theme but it does look like CON

1. Manpower would significantly be higher due to the population boom after the two world wars
   
2. Wars would be drastically more destructive, especially with the use of nukes
   
3. Anti-war movements would probably be started in countries at war
   
4. Best case scenario would be small countries bullying each other
   
5. Worst case scenario would be big countries harassing each other
   

Lets say the world will not be wiped out after the war.