* I want it to know that someone can only do one task at a time (I don't see how to do this with Smartsheet, at least). * I want it to know that unassigned tasks are blocked on people finishing other tasks and becoming available. * I want to be able to see how long things will take if I add or remove N people from the project. (For example, if only one person is on the project, the total project time should be the sum of the time of all the tasks; if enough people are on the project, the time to completion should be the length of the longest sequence of dependent tasks.)
I am surprised this doesn't exist, or at least that I can't find it (it's a feature and not a product, but feel free to take the idea and run with it if you want). AFAICT, the major project management tools in this space don't do this. But I hope to find out I'm wrong. :)
You should read The Mythical Man-Month. If that doesn't persuade you, lots of people use Microsoft Project to do what you describe -- generate Gantt charts that only bear a passing relationship to reality and prove incrementally less useful for estimating project completion.
It should be able to do all of what you ask although I don't it will do what you ask in your third bullet point unless you specifically assign extra people to tasks.
However, as others have pointed out, how much a task will benefit of extra people is really something down to domain knowledge and experience and the tool is just that, a tool to help you.