As others are saying, you have a pretty good start already. Having a bit more consistency over the range would help. You have a few different typefaces going on, where just 1 or 2 and using them the same way on each module would bind them together.
Some custom knobs might help you out, too.
You may even think about keeping a similar background colour, but not needed. I’m always a fan of the dark grey, though your greyish blue one might be a really good colour to give your modules your own personality.
I would at the least recommend a logo for your “brand” of modules to be put on all of them.
If I have some time in the next few days, I’ll see if I can put an idea together. But I would heavily push you to try it yourself, as you’re already on the right track. =]