I humbly submit that to alleviate the sparseness of players in some Fleet Actions and PvP tiers, we should flatten the world. That is, use the existent squad technology to adjust the level of all players that enter a given Fleet Action or PvP queue to some target level and allow them to all play together. By eliminating tiers and level banding, this would increase the population in these instances across the board as well as add a new and interesting dynamic in the diversity of ships and abilities that would be brought to the fight.
If there's concern that the contribution of players level adjusted up or down by a large amount would be skewed, you might consider boosting or penalizing player contributing based on real level vs. target level, but this is an exercise for the designer.
I think that in Season 2 they're going to add rewards for playing "Fleet Action Whatever" NOW. So basically you'll get an assignment to play in a fleet action and by playing it when they tell you to, you'll get cool loot. This will increase the number of people in a fleet action instance since much of the game population will be trying to get the neato gear. WHile not the same thing you're talking about, it should help alleviate the sparseness.