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.
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.