The Developers behind Ship of Heroes are working on the code for missions, and this reflects their plan for the mechanics of how missions should work. They thought that perhaps they should start describing what they have in mind so that all of you can weigh in before we settle on this approach. Each of these factors is more or less independent from the others.
First, they still plan to let characters set their difficulty level for most missions, so if you are level 20 you should be able to dial the level of the mission up or down and get level 21, or level 19 enemies to fight. And perhaps a few more levels as well.
Next, they also intend to allow characters to affect (perhaps with a slider) the composition of enemy mobs. So you want more trash mobs? Fine by them. More lieutenants, bosses and maybe an Arch-Villain? Again fine by them.
Enemies need to scale to the size of the hero team, so a mission that has a solo hero obviously needs to spawn fewer enemies and lower level enemies than the same mission being played by a team of 10 heroes.
These factors do all interact, and we are planning this out right now, in detail. And some mission types begin at higher levels of difficulty than others. So any given mission can deliver a totally different player experience, depending upon how the player makes his selections and forms his team (or does not form a team).