my suggestion is that for a sandbox game you need to have infinite ressouces. So iron and gems dont run out. It is really importend if you dont want to fall in the same gap as Timber and Stone where the ressouces got really strong limited (i know that is supposed be hard if you try the actuelly version you will know what i mean)
So i think a bit about it an came up with some posssible solution:
You could create deposit (some kind of structur over the ground where you position you mine of them) so you build a get infinite ressouces out of them. To make sure that game dont be
to easy this deposit will be under the “Box”-system so first you need to
clean up the area and protection when the enemys come back.
Smaller and not infitive despoits you can find more easiere over the map and
make sure you can equip you army properly.
Or you can make a option
when you start a game that deposit are infinite or not. The downside
of this system is you need to have lot of despoits over the entire
map and digging underground dont make any sense anymore. (at last
not for the race that live above the groud.
You add some kind of trader in this game so you can exchange the
ressouces for limited ressouces. Downside of this is that is not anymore a sandbox game if ressouces are not infinitiv
Or you could make somekind of job that create rar material such as iron and gems. Like a Alchemist.
So my option is if i want a survive game i would go an buy Timber and Stone or something like that.Sandbox games like. Minecraft, Castle Story, DayZ all one thing in common they have infinite supply of building material so make sure you have that her too. Because that would create make long-term motivation