Whatever happens, don't bring obligatory endeavor rule back - Having to take one ship specifically in order to be able to take another is rarely a good idea, as it forces too much of a fleet composition on the player.
1/500 wouldn't be so bad, though, if we're really afraid of AC spaming. That would mean Endeavor has no restriction, Endurance 2/500, Defiant 1/500. Seems like a fair progression.