So I was thinking a bit about the retirement and prime runner ooption and just a higher grade of runs in general.
First off I'll say this I LOVE the Shadowrun Missions system. I liked Denver but I really enjoy manhattan. I like that New York you get the feeling that your being taken seriously as a character. I got really sick in Denver of being told at the beginiing of a mission "Oh your desperate for work this week". I think allowing players to choose their level fo difficulty is good.
But some people have raised the point of some characters being too experience for "Milk Runs" and other runs being too big time for what people are being offered money and karma wise. I believe there is room for a tier system but my recommendation is having a Low Tier and a High Tier. One thing you have to be carefull of, people come to cons wanting to try the system. I would make the recommendation that you should always allow people to play up, with the understanding that the lethality, and the pay offs are considerably higher. From my limited experience in SR Missions there's not that huge of a difference between TR 1 and TR5. A higher and lower tier would allow greater spread and greater opportunities to tell stories. If you limit high characters from playing in low mods it encourages them to make back ups, but keeps their old beloved characters viable.
That's my suggestion at least.