I'm not going to necessarily disagree with the person above, since your open-ended goal will almost undoubtedly lead you to think about more stuff. A Mantis case is not a bad idea for an ambient situation, and even if you don't fill it completely, throwing some blanks in there and calling it a day will make it nice and spacious so things are easier to access.
However, I am a believer in small setups if they are focused, and I did not go the large interwoven case route (yet). There can be advantages to different smaller systems if you're organized about it.
My main suggestion is to not bother with in-rack effects in a synth setup this small. Save that space for LFOs, maybe a Sample and Hold, and an attenuverter/polarizer. Maybe another VCA that's more for CV. Focus on the stuff that makes modular modular for now and then just get outside effects. If a pedal or something has an expression or tempo input, you may even be able to modulate the pedal with CV if you're careful and look up the proper methods.
My other suggestion is to go all out on your filter. If you love the Forbidden Planet, go for it, but it's a bit workmanlike for an ambient setup, where texture makes a huge difference.