This was really tough, because I am currently downsizing a bit myself. From a purely cartridge point of view, with no regard for your current inventory, my choices would be
.22 LR- not a semi do to some local restrictions
12 gauge- a break action, again because of legalities
.270- the world's preeminent open country round, not restricted due to military use, it is used &
available everywhere
.338 Win.- the ultimate N. American safari cartridge, accurate & reasonably kind to the shoulder
.404 Jefferys- simply because it's a .404 J