My idea is very similar to what Ian suggested and what Keg expanded on. Progression over a long period of time should give you ability to use more troopers per run. Maybe once you reach level 100 you can then take 2, after claiming a certain amount of achievements you can take 3, possibly open 1 more slot when you claim your Grim Reaper Eddie, etc. It just makes sense to only be required to play 10 or 15 runs daily after you have played for a year plus on a daily basis. Some of us are still going to run levels 50-75 times per day gathering materials anyway, It just shouldn't be the obligation that it becomes for some players.