First of all, no country should ever be allowed to build units in a captured enemy capital/factory/production centre in any Axis and Allies game ever period.
I will add the Munich factory next game, this is for me just ahead of the other candidates Ruhr and Alsace.
Benefits of Munich are that its sufficiently well behind the front line to begin with, but allows every German home region to be reached by newly built units in one turn.
It also allows Germany to reinforce Austria’s drive in the south.
Will probably also make Petrograd a production centre and Russian capital (though Russia can move the capital to Moscow once in a game.
I’ve considered Canada as the magic 12th PC, perhaps with reduced starting units.
A completely different approach to placement is:
New units may be placed in any home region still contiguous with the capital subject to the following limits each turn:
Capital: 3 units
Controlled home region: 2 units
Contested home region: 1 unit
In addition
UK - Canada: 6 x IPC of units
UK - India 6 x IPC of units
UK, F, G, I - Africa 6 x IPC units (Askaris only)