Yeah I thought extending to all territories.
And I also thought of ncscswitch’s comment.
Its easy to argue why realistically only enemy Capital require this.
We could consider extending to all Victory Cities, but that might be too much too.
What we could also do, is that conquer Capitals goes back to original control if not occupied by you have no land units there.
And I just realise I didn’t finish typing the draft rule.
Conquer Capital
A captured capital territory generates 50% less income unless controlling player occupies it with sufficient number of land units. The number of land units required is equal to half the number of IPCs in income.
Capital Territory      Units
Germany/Berlin      5
Japan/Tokyo      4
Russia/Moscow      4
United Kingdom/Londom      4
Eastern United States/Wasington DC Â Â Â Â Â 6