THIS WON'T be official battle royale application, but for a normal map.
I came to idea to make a map based on Tribal War - Florida... with one different rule - you have your "protected" start spot, but you lose if you lose all your territories in the open spot.
Something like this - rough sketch
So you get random deployment in the wild (green) and one starting territory (red) which has auto deploy and can't be attacked. Losing condition is not having any troops in the green.
I guess it could work for all type of games. Tell me what you think of gameplay, then I can attend to making graphics.
Of course, in the green field, everyone can attack everyone.