Christie is ethically challenged (which those of us on his home turf have known for decades). He can't help President Trump -- is much more likely to hurt Trump, if he's given any real power.
How about an ambassadorship to Lichtenstein?
He’s going to step down from being governor for an an extremely minor role. I don’t think so.
If there's dirt at the FBI they could use Christie to smoke out some more leakers. Give the press some raw meat to choke on for a while. Leak various juicy tidbits about past or present FBI investigations and follow the leakers. When he outlives that bit of usefulness he can resign. Meanwhile the Trump train will keep chugging along.