My fantasy choices:

.500 3" NE - H&H or Westley Richards sidelock - for the big stuff close up.

.450/.300 3" or 3 1/4", scoped in QD mounts - as my general 'medium' bore double rifle, larger plains game or plains game as needed. Perhaps even a modern made DR in .500/.416

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.300 H&H Mag Johannsen (or similar) M98 takedown, scoped - plains game rifle and world medium game hunting rifle. Takedown so it fits in a nice DR sized case. Perhaps a second barrel in .375 H&H.

Purdey 12-bore side by side - for some relaxing bird shooting between the 21 day elephant safaris my rich Uncle will pay for me. After all it is no good having all the hardware without having safaris to use them on!