Not sure I want to Expand bsyond this Point.
What are you missing?-- Cangore
You may have answered your own question. Your rack seems pretty complete and looks pretty fun. If you're happy with it as is, I'd say leave it and enjoy it (and save the money!).
What are the goals with the synth and for expansion? It looks well-laid out, depending on what your aims are with it. Maybe switch out the passive mult for a buffered one, so you can use it more reliably for 1v/oct? You could consider a precision adder for manual control over transposition (I love the vpme.de T-43 for this), but I see that you already have MIDI connecvtivity so maybe this point is moot to your workflow. Really, other than suggesting smaller versions of modules to squeeze a few hp out here and there, I don't think there's need to change anything.
Enjoy the music-making!