I put together some ideas in my "Combat Modes and Starfleet Ranks" blog entry to address this issue.
The basic idea is that if you key gameplay to rank, then players will voluntarily choose not to be promoted further once they get promoted to the rank that brings them the kind of gameplay they enjoy most.
Here's a picture showing how I'm thinking the ranks might best be sorted by gameplay content type:
To make this work, I propose three things:
1. Make the gameplay encountered in the first ranks (Ensign, Lieutenant, Lt. Commander) about fast-paced, exciting, hands-on tactical action; make middle rank (Commander, Captain) gameplay about organizing players into groups and leading them to complete operational goals; and generate gameplay for the highest ranks (Admirals and perhaps Commodores) that calls for logistical thinking and long-range planning in pursuit of strategic aims.
2. Let players decide when to stop accepting promotions. If they like tactical play, they can stop at Lt. Commander. If they enjoy leadership, they can stop at Captain. And if they're into thoughtful strategic play, they can keep going until they earn promotion to Admiral.
3. In addition to allowing increases in rank, also allow players to become better at whatever their current rank is. If someone wants to stop at Lieutenant because they like the kind of gameplay generated for them at that rank, they should still be able to become a legendarily good Lieutenant. The kind of gameplay generated for them would remain the same; only the difficulty level would increase to meet their increasing level of capability at that rank.
I believe that if you put all three of those features together, you'd find that players would be happy to stop at the "lower" ranks because that's where the content they enjoy most could be found. Since most gamers prefer tactical play, and some gamers like leadership, and only a few gamers are into strategic play, the number of players would naturally wind up smaller at the higher ranks. No arbitrary limitation on how many of a given rank there can be would have to be imposed by the developers. Instead, players would voluntarily sort themselves out into a pyramid with lots of players at the early ranks where the action is and only a few players at the big-picture ranks.
It's likely that there'd be some players who would ignore the association of rank with gameplay content out of a belief that they "have" to have the "highest" rank. For these folks, it might be helpful to allow voluntary demotions so that they can return to the rank that offers the kind of fun they really prefer.
I'd also suggest (and notice that my illustration above follows this idea) that players wouldn't always just get one kind of content created for their rank -- they'd also see the next higher kind of content so that they could try it to see if they like it. If they do, they can accept promotion once they've earned it; if they don't, then they can refuse promotion and concentrate on improving their skills at their current rank. It's sort of a "try before you buy" feature.
So how about this? Would something like this work to address the concern that there'll eventually be too many Captains and Admirals and not enough Ensigns to order into the nearest Jeffries tube?
