I figured this would be better off here in discussion than in suggestions, since I'm not really for or against it at the moment.
Currently, legendary status is simply a step up in standard skill progression, allowing people to eventually get a fortress full of rather unremarkable legendary dwarves.
Why not cap normal skill progression at grand master, or whatever the thing is directly underneath legendary? In order to attain legendary status, the dwarf has to do something that deserves notice. This could be s mundane as creating a certain number of masterpieces, or forging an artifact. For military dwarves, they could train up to grand mastery with whatever weapon they've got, but in order to get to legendary status they would need to defeat a certain number of goblins, singlehandedly kill a megabeast or some other impressively fighty thing.
This would make it so that the less-skilled dwarves could have a chance, however minor, of reaching legendary status. But since having a higher skill will increase the chance of masterpieces and lengthen the amount of time one can spend fighting with a megabeast, higher-level dwarves would still have an easier time getting there.
This is just a rough framework for how a system like that could work, and I thought it was an interesting enough idea to share. Whether or not it actually belongs in DF is another matter.