I've made a bug report already, but since this DD is about theaters here I go again. Could you *please* stop theaters from spilling over into your puppets and/or allies territory? This doesn't happen for majors, but it does for smaller faction members and puppets that share a land border with the larger country. Germany, China, and the USSR are good examples.
I'll use Germany and Croatia as an example first. Say Germany beats Yugoslavia and forms Croatia. The theater covering Croatian territory will be a German one and not Croatia's theater. Not a big deal in this case.
Now, let's say I mod Xibei San Ma to be a Chinese ally instead of puppet (they defected to the communist side after Japan was defeated, so an alliance makes more sense). Same deal as before, China's theater now covers XSM territory instead of a XSM theater. Now, this causes problems when you try to unite China and break that alliance. Since you can't affect foreign theaters, even though the two aren't allied anymore, if you hover over a XSM province in the theater map mode or supply mode, you'll see that it's supplied from Nanjing and part of the Nanjing theater even after the alliance is broken. That causes problems with both nation's theater AIs. Because any objectives placed inside XSM by China is considered "in friendly territory", the AI won't man that front if you go to war, nor will it attack. The exact same thing happens with the other Chinese states if they were ever at any point allied to Nat. China. That means you can't put your armies on AI control, as the AI controlled units won't do anything. The same happens when those nations are on AI control; they'll just stare at each other forever.
Could this be fixed? Each country should be covered by its own theaters, not those of its allies or master nation. It really, really, screws up trying to create a continuation of the Chinese Civil War after Japan is defeated, as all the warlords were allied to each other and can't attack the others because their theaters overlap.