"Let's say you decide to run away, then what?"
Translation:בוא נגיד שאתה מחליט לברוח, ואז מה?
That's an interesting question. It's not a grammatical issue, it's a nuance of style. You could say בוא נגיד שתחליט לברוח. It's grammatical and understandable and means exactly the same. It's just not used as often. I suppose it's because בוא נגיד ש is followed by a hypothetical situation, and we usually use the present tense for things like hypothetical situations, describing the plot of a film or a book, jokes. I think the point of it is to make the listener imagine his or herself as experiencing the situation as we speak. It is also the case with using the second person singular. This sentence could mean: suppose you decide to run away, but it can also mean: suppose one decides to run away. If you say: בוא נגיד שתחליט לברוח is more specifically a practical question about the intentions of the listener.