@Lugia Power really is the next upgrade, and ensuring that I have more power than I need.
The List and other people's suggestions helped.
"What single module would add the most to this?" The single module that would give the most wide-ranging enjoyment, the single module that would expand the possibilities the most.
Since adding Beats into my case with the DNi Pro Dot + ADDAC103 T-Networks I've been enjoying much more beat and rhythm based music, I now need to take a step back and experiment with more long-form ambient with what I've got. A lovely moment was discovering what the ADDAC103 was like going through granular processing in Clouds without the reverb wash and lovely step away from beats with a creaky, glitchy and slightly spooky sound.
Other sensible additions with the most advantage would be:
Arturia Keystep - so I can get a bit more hands-on with the notes.
Pamela's New Workout with Pexp-1 Expander - so I can properly sync my external gear (While adding re-sets across my rack) while still leaving the Disting for Sample Playing.
I think Pam's has to be next after a case power upgrade.
Enjoy your spare HP, don't rush to fill every last space, this is not like filling sticker books. Resist the urge to 'complete' your rack, its never complete so just relax.