The possibility to assign variable in any action and his value be watched without other assign outside the action
Do you mean something like Site Properties?
A session could be a better example.
Sometime we have a situation where a variable created in a specific action need be changed inside another action, but to continue in your flow, you must take the value of the new value in the output of the action and assign the variable.
We could use a session, but the session have a low performance because need use the database to handle the data.
So, the ideia is we are able to define the variable as global to be used and modified by any action, it include in action in other eSpaces. I understand that it is very complex if we think in the .NET and Java side. Maybe Outsystems could apply the assign automatically in each action that use this kind of variable to be possible this ideia.
Since you are comparing to session variables, do you mean it doesn't need to be persisted? For the use case you have in mind, would it be acceptable to close the browser window and lose the value, for instance?
Yes, the idea is not be a session, but a comum variable as a global that have life only in memory, during an action execution, but could be assigned in any action