Reading your question, my first thought is that you are describing a classic workflow driven approach. Basically, you want to let the workflow engine decide what code path to follow. For instance, suppose that you are processing a request for an insurance quote, your workflow might look something like this:
Enter details > Compare details to risk matrices > Generate quote
That is the "happy path" through your system. In reality, your workflow is going to contain branches where you need to go off and do something else; for instance, if the risk matrices indicate that this is a high risk customer, you might want to route the quote request to another department for manual intervention.
Now, not all workflow based approaches are going to require you to operate off on long running tasks, so it is perfectly acceptable to create workflow chains that suit your needs. However, most workflow systems have visual designers that allow you to chain your rules and tasks.
You know what I'm going to suggest now don't you? Yup, it's time for you to go off and research some Workflow engines. I would start with Microsoft's Workflow Foundation (it's freely available and there are a wealth of training resources around on this).
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