In my opinion, I think default map for Sandbox gamemode should not be an island.
Here's why I think so:
-A lot of vegetation is obstructing player's view.
-A lot of bumps and objects in the way (especially not good for vehicles).
-A visual cluster.
-Generally not a lot of space to roam around.
-(Probably) uses more hardware resources than needed.
So here's what I'm suggesting:
-A map that is visually appealing while being centered on being flat.
-A map that offers alternatives for testing various functions, such as water, underwater, hills, sand, snow, ice, high altitude and clouds (even space if possible), etc.
-Some structures such as airfields, a few tiny towns, a few asphalt roads, etc. (for testing out, but it's not the most important imo)
-Destructibility would be a quite a nice addition too.
-A pretty big size map in general, to enjoy high speeds from vehicles such as cars and aircraft. This includes big heights too.
-Flat areas should avoid high bumps or not have any bumps at all, in order to enjoy using high speed vehicles better, such as cars and aircraft.
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In other words, it's something like Just Cause-ish, but without visual cluster or buildings/infrastructure everywhere (nor humongous size obviously).
A pretty decent example would be a map from that unnusucessful sandbox game where there is a big flat desert area surrounded by mountains/hills.