Thank you for those great insights !
Indeed I’d like to be able to blindly run terraspace all up.
It would be great if we could decide the instantiation of a stack at the stack level.
For instance:
stacks/db/base.tfvars contains count = 0
stacks/db/prod.tfvars contains count = 1
This way, at a glance, you can see that there is no db instance in dev environment, since no dev/tfvars file 
Since Terraform 0.13, count metadata is available in modules and could solve this, but at first sight, it cannot be set with tfvars.
In my current setup, in the dev module, there is no instantiation of the db module (no db module declaration).
Another issue I’m facing is that, sometimes, there is a stack common for all environments. Let’s say I have an ElasticSearch cluster (a quite expensive service !) and I want to instantiate this stack only once.
Could it be solved by creating an all or common environment, instantiating ES in this environment, and access it this from the other environments with remote_state data sources ?
When I use the output helper, say output('vpc.vpc_id'), I understand this as fetch the vpc_id output value in the vpc stack belonging to the same environment i’m running the output helper, is this correct ?
Perhaps it would make sense to have an option like this:
output('vpc.vpc_id', environment: 'common')
Sorry for all those questions !