There is one option that you stated. If you are really concerned that a person may drop, or may come back at a different time/level, then create the game with 99 level cap.
I make mine 1-130 level cap everytime I play, it just makes it so much easier, not only that, if there is a person solo'ing at higher level, and you only have 4 in group, that
person will see your game pop up, and may ask to join with a different character.
I don't think it is needed, just my point of view.