Just don't make them timed meaning that they are only available this week etc.
Also they better be somehow tied to the story.
How is the new Hitman btw? I'm waiting for the full game to release. Absolution lasted 15 minutes before I uninstalled. Loved all the previous ones though.
I bet the more you collect intelligence, optional missions will also come up. From the trailer though we don't see many different mission variations. I hope they're a bit varied and complex sometimes.