A few suggestions in addition to the excellent suggestions that Lugia made:
Switch to an Intellijel 7U case and put the noise, S&H, Quadratt, and audio I/O, perhaps MIDI module, if you need it.
Do you need all the functions of the metropolis or would a Varigate 8+ get you sorted?
You do need an envelope generator. Consider the XAOC Zadar or Malekko Quad Envelope.
Look at the micro version of certain modules, such as Beehive from Michigan Synth Works instead of Plaits.
The signal mingler looks interesting, but again quite a bit of HP. While not exactly the same, the Klavis Mixwitch does a good amount of the same, but you could add the Logica XT on top of it and get more out of it.
I know Lugia is not keen on FX modules, but I do like them. The question is always: how much control do you really need? Do you just want to add a little richness by adding a touch of delay and reverb, or do you intend to use it as an actual integral component of a patch where the FX changes in character? For the latter, you do need more control. For the former, consider the 2hp Delay and Reverb, or the Pico DSP. The Black Hole DSP2 is also nice and offers a bit more control and stereo. 2hp is hard to beat as a utility FX.
A nice mixer for your setup would be the XAOC Praga.