I like the idea of neutral countries that can only be activated by specific powers; for example only Russia can activate Mongolia and only the US can activate a neutral Mexico and Brazil (heck include Liberia) once they are at war on the respective maps. It’s similar to the special custodianship ANZAC and UK Pacific have with Dutch Territories on the Pacific Map. As I am aware, the US cannot put a control marker on a Dutch territory unless the Japanese (or another Axis power) took it first.
Unless they’re attacked the first turn, the US will lose 3 IPC but it’s not a huge hassle to activate Mexico once at war.
As mentioned previously, Sierra Leone should be part of the UK already.
Persia should be pro-Axis, but only if USSR’s Spread of Communism bonus was modified to focus on Continental Europe (could just make it original German and Italian territories). To make up for this you could add another infantry in West India I guess.