How about multiple occupancy modules for moving several people (5-10, or more?) but for very short distances (or else they'll get very angry).
Module degration would make customers unhappy (Re; squeaky door, defective TV etc..).
Different grades of galleys, basic vending machines (low maintenance, too) to, say, replicators (medium maintenance) to a cook (high maintenance, and food cargo could enhance this).
High-capacity cargo pods and fuel tanks (but would also require better maintenance).
Each upgraded module would also be heavier than the previous one.
Engineers would also have skill levels (like they do now). You could also have a equasion on their chance of being able to fix an issue, Hmm, something like ((skill-difficulty modifier)*10)%, so if your vending machine breaks (say difficulty 1) and your engineer has a skill level of 5, then he'd have (5-1)*10%=40% chance of fixing it, while if your engine breaks, a difficulty level of 8, your level 9 tech would have a 10% chance of being able to fix it. Of course, a level 5 engineer would have no chance (well, -30%) of fixing it. The engineers could also maintain the modules so they do not wear down as much as they would normally.
How about research quests to unlock the higher end modules, you need to get these 3 components to this planet for them to be able to invent the luxury suite module.