Based on your comments, it seems that you are trying to introduce conditional uncertainty. I'm not certain that you can do this by simply adjusting that verb. For example, you could introduce uncertainty by switching it to "may", but I believe that is not the meaning you want. "Project milestone may be missed" ...but it may not. There's no great impetus.
It is my opinion that the meaning you are after will almost certainly require more context. That is, you would qualify it with another clause. "Without x action, project milestone will be missed". You could, of course, combine them: "Without x action, project milestone may be missed", but this creates a sense of less urgency.