The todo.txt file format supports extensions, and some of the extensions include start date, due date, recurring tasks. The Simpletask todo.txt Android client supports these, and can filter by things that are past their start date, sort by due date, etc. Todo.txt also supports projects/contexts/lists/tags and Simpletask can filter by them.

i don't know which todo.txt commandline clients, if any, can do this but i bet some can (topydo maybe?). Simpletask supports Lua scripting, i dunno if this is sufficent to allow dynamic prioritization of tasks, but it might be. I don't think Simpletask supports dependencies but topydo does.

todo.txt is a file format. The nice thing about todo.txt-based tools is that the todo list is stored in a plaintext file in a relatively readable and editable format, with one line per task. This allows you to edit it using ordinary text editors, grep it, and sync it using utilities (resolving conflicts on a line-by-line basis).