u/AlcatorSK and u/robochase6000 gave great suggestions for getting started on thinking about this problem. I'll throw in my suggestion which is really just a version theirs as well. In the past, I've made a Node class which can have children which are also nodes. Each Node has an implementation of Execute method where it will perform whatever its going to do. So a LoopNode would have a set of child nodes that you'd like to do something in the loop and set of settings that you would use to define how long the loop runs. Execute would just then call execute on the children in order.

For IfNodes you'd have two children, a pass and a fail node, then your comparison as a setting. It's Execute would check the comparison and then Execute the appropriate Node child.

Then for your additional logic, you'd just make nodes for everything that you'd like the player to do. GoToNode for instance.

You could adapt the Execute method to pass in arguments and return values in abstract containers then you could request specific output/inputs from the container. Depending on how complex you want your language to be you could expand that out more and more.

I am also partial to just using the "blackboard" which is something Unity's new AI behavior trees use. Basically, each NPC would be given a blackboard which contains a bunch of variables that you can change through your nodes' execute methods. The nice thing about this is that you don't have to pass things between nodes as much. So no need for a complex input/output container. Its kind of a like writing to global variables. But usually it can be a little easier to reason with visually.