Ok how about some ideas of a seniority system that has some modifications that can benefit junior crew?
Here's one:
A certain amount of hours per year (or other predetermined time) that could be bid for in reverse seniority, before the seniority run.
This would probably work best with a one trip per roster limit.
Example: 200 hours per year, so this might mean 1 LA a roster or 4 JFK shuttles a year.