+1 for some of the mutable engines. Plaits is a very versatile module.
+1 one for an additional reverb. Something “lush” capable of long tails would nice but I guess that is quite CPU intensive.
How about a simple digital Buchla type oscillator configuration? That would include 2 OP FM with added wavefolding for the carrier. A foldable triangle and/or sine wave carrier plus sine/triangle/… modulator and a lowpass filter plus routable, loopable envelopes for the operators’ amplitudes/ folding level/filter?
What I am missing a bit in the current engines is a filter envelope, vibrato (very important) and a one shot (triggerable) sample and hold mode for the LFO. Personally, I find yet another digital subtractive synth engine not so interesting. The OP-Z is clearly digital, so why not taking advantage of that and trying something different?