First, I 'd like to thank StevenO for his comprehensive answer. I'll try to answer his concerns and questions as best I can. I will also adress Zuko Mal in a separate post later, his input is most welcome and I will try to respond shortly.
In all fairness if you're getting it in Crimelord we should keep in mind that a "5th-tier" talent only requires 5 levels where it takes 9 levels (or creative/lucky multiclassing) to reach that level. [...]
I believe that the legendary 5th-tier talent require 5 levels of the PrC Crime Lord to reach, and 7 base class levels before that. So that is at the 12th level at the earliest. How to do that with 9 levels by your counting is a mystery. Using NH-levels you could do so by CL 8; a Noble1, NH6, Crime Lord5 would do.
Defensive Circle (Jedi Knight), Full Control (Droid Commander), Nowhere to Run (Bounty Hunter), Shield Gauntlet Redirect (Kilian Ranger FU tradition) and Uncanny Dodge II (Scout) are all 4th tire talents. The utility of these talents varies greatly, but you are most likely not seeing these very often in use due to the high level and focus required. Defensive Circle can be pretty good, at least in a party with multiple Jedi or Lightsabre wielders.
While I'm far from sold on the idea of more "master classes" (sorry but 2nd tier PrC doesn't seem all that genuine to me) if you're going to create them they should actually be very limited. By this I mean what makes the "master force user" classes really special? It needs to be the Force Secrets as one could (and some have) argued that the Tradition talents are already available to any Force Sensitive even if it requires jumping through what are often roleplaying hoops. What's a defining feature of Force Secrets? Most (All in the SECR) require spending a FP to use for effect or a DP for a greater effect meaning they all burn through what could/should be a limited character resource. This leads me to think that your major mechanic should also burn through the limited resource and here I guess I'd ask what you think may be appropriate.
This helps me quite a bit actually! Force secrets is all about power, at the cost of FP or DP. Access to Tradition talents is flexibility, it lets you mix and match abilities that may be difficult to get depending on the era of play (and you GM). It is also about discovery, letting you learn “new” unusual talents. But all those secrets and talents are about the force, any new 2nd tier PrC should be about anything but that.
There should be at least 3 PrC that grant access to any new 2nd tier PrC, or possibly gaining access by going deep in a single base class. They should have access to one talent tree of each PrC that grant access to the class, possibly one new tree as well, thus granting the flexibility the Tradition talents. Any new talent should fill a hole or add something new to the game.
Any Special Ability or Secret should be unique, not coming from a list available to other 2nd tier PrC. They can be fixed to a level or come from a short list. A list gives more flexibility, like expanding the uses of Share Talent from the Officer PrC or letting you apply the effect of talents on your allies when using Lead Infiltrator. Those effects probably don’t belong in the same 2nd tier PrC or list…
Another possibility is to expand the usefulness of Persuasion and Deception, for example with a Special that let you use said skill(s) on a group. Maybe you can persuade your Cha-bonus number of people at once? Double it with a FP and even more with a DP. Another Special Ability could reduce the pesky penalties that are associated with those skills. A minus 5 costs a FP and a minus 10 costs a DP for example. Normally you can’t make more then one use of these skills in an encounter, at least not against the same target. Maybe one of the Special Abilities of Secrets could let you attempt a skill more than once in an encounter, at a cost.
A sniper/combat/ranger 2nd tier PrC could have a Special Ability or Secret that let you attempt an impossible shot. Maybe you could shoot a bit longer than the weapon normally could reach, picking of a target from 1,000 meters with some luck. Or use a weapon to manipulate an object, like shooting the TV to change channel or close a blastdoor, call an elevator or whatever. Now go watch that episode of The Simpsons…! Now, maybe some GM would let you try this at the cost of a DP or a FP, but then it is not something that you know you can do. They could be called Weapon Tricks.
These are all just a few examples, not at all ready for play. They would need to be refined and adapted to fit a specific 2nd tier PrC.