Percentages don't work when it comes to free projects. Then again, if you were to have dead-lines, it will be Duke Nukem: Forever and StarCraft: Ghost all over again. Both games were constantly changing design approaches, which ESF is doing all the time, though not as much as the games I mentioned.
Here's an example of how dead-lines aren't all that great:
A modification for Half-Life 2 called Eternal Silence. It's an awesome modification but it is extremely laggy and buggy, much more than the average beta release.
About the trailer idea - It's not a bad idea but it would be better to release one the day ESF 1.3 is released, that way those who are new to ESF can decide if they would want to try it.