The right module next for you is the module you want. I don't understand why guys are asking other people how to design their rack or which module to buy.
-- GunnarWaage
I get it. I like to have others' thoughts when I feel like I need to make a decision. Each of us can always learn something. I think asking others is just part of the learning process. There's always going to be sombody else out there with more smarts and experience (at least compared to me :)~ And, I'm going to want to hear others' experiences when I'm undecided about dropping the kind of cash that a Eurorack system can soak up!
i already got all the modules shown in the posted racks
-- Broken-Form
So, like others have already asked, what's not blowing your mind right now? Are there things that you use all the time? Do you have any modules that you like but have a hard time actually using? What makes you think that you need another module, other than a desire to fill up empty space? Do you just want some blind recommendations, or are there things/sounds/effects that you want to learn how to create?
I think that's what I take away from Gunnar's comment, above. If you want useful recommendations, it helps to have some idea of what you are looking for. IOW, if you don't know where you're going, you're probably not going to get there. And I say 'probably' just because even a blind squirrel gets a nut once in a while.
So, to my point, here's my suggestion... Since you want to do dark ambient, you may not be interested in adding some percussion but, since I'm not yet sure what you're looking for, that's what I would miss in your rack. Personally, I can't imagine having a rack without it. You may not want a 4/4 kick or snare hits cutting through your dark ambient atmosphere but it would add creative possibilities for adding punctuation and playing around with tempos and rhythms.
At the same time, there's nothing wrong with not having any percussion modules in your rack. Be bold! Do your own thing! Best of luck with your journey!