Interesting suggestion of a sampler. Is there a small portable sampler that is polyphonic so can do chords?
I have a microgranny, which I love, but it can only playback one sound at a time and only one instance of that sound, so no chords or two different sounds monophonically. A poly granny would be great.
MC 101 and Digitone do sound good. 101 perhaps more varied, but Digitone better for making your own sounds. I need to listen to Circuit Tracks.
I think the 1010 blackbox can multisample. The new 404 has some poly abilities. You could perhaps just sample the chords you need.
The digitone is pretty amazing though.
Surprised no one suggest the Analog Four. The MK1 is portable and fairly affordable second hand
1010 Blackbox does look great, and can trigger polyphonically. I need that if it is going to ‘mirror’ the OP-Z note for note but with thicker and richer sounds. Good form factor too. It is nearly the price of a digitakt though.
The benefit of a synth is a modulation matrix and being able to change timbre characteristics live. That is tricky to recreate with a sampler, I guess.
On the plus side, a sampler opens up possibilities with loops and granular (which intrigues me).
The OP-Z can sample into synth tracks and be played polyphonically but any time-based modulation changes with pitch which often isn’t desirable, and there are limited rooms for samples: only 10 per track if you ditched every engine on the OP-Z. About half the engines are decent so only 5 sounds for all songs.
Ultimately this quest is about sound quality/flexibility/power of the OP-Z and mixing external gear in with OP-Z and it not ‘sitting’ right (I think to do with compression or lack of for external synths), and making the most with a minimal portable setup. (throw in a rucksack on a work trip). So I’m thinking OP-Z as sequencer and drum machine with two bi-timbral synths and some mini mixer. I’m asking a lot, but have seen it is possible with MM2. Interesting how many in the thread have stated its quality. And also interesting there are several grooveboxes in the suggestions.
These are great suggestions in the thread. I should update the first post listing them all…
I wouldn’t be surprised if the upcoming TE module has midi out and stereo in. An advantage to one of the new boutiques could be the ability to run the MM2 through its inputs, then feed its output into the Z.
So many hopes and dreams pinned on the new green module in the OP-Z community 
I use usb-c with a hub for midi and I have a DigDugDIY trrs splitter which allows audio in and out for the module track.
Let’s see what they release. Quite possibly something unexpected…
Yes an external synth module with audio in would be very beneficial. Good shout on the boutiques for that.
Not sure if they’re that much in the US but over in the UK they’re about £500 second hand.
How come that no one posted the most obvious choice?
Bi-Timbral, portable AND affordable…
Korg Microkorg!
New for 349€, s/h for 250€ or less.
It’s battery powered and bitimbral.
Apart of what @bibenu said I would point out global stereo effects. So you could pan your bass and lead tracks left and right to process them separately but if you use Cycles own reverb or delay you will hear it in both channels.
You can kinda low pass filter Tone machine with Sweep and Contour. However Cycles is very bass heavy on its own and doesn’t have anything on board to tone it down. I run Cycles into Digitakt and wish DT had EQ or filter on inputs.
So it’s doable, but feels like a temporary solution for Cycles owners until they get a more flexible synth.
Ok, I gave Cycles another shot and this is what I ended up with. All sounds apart from drums are Cycles tracks. One track uses Cycles’ delay and reverb. Other melodic tracks are sent through Digitakt’s delay and reverb. Bass track is dry. Digitakt compressor on top.
There are 3 sections:
I’ve used a ton of delay and reverb in Digitakt to spread and smear Cycles’ mono sounds into underlying pad/texture.
Originally suggested the microkorg but then deleted it because I was a bit iffy. They can get away with calling it bi-timbral, as each layer is a full synth program, but as far as I can tell the layers can’t be addressed with separate midi channels; SOS review appears to confirm this.
Apart from that it’s a badass little synth. OP should buy two of them.
Edit: Microkorg XL (and XL+) can have separate midi channels for each layer.
Mutable instruments - Ambika
nice now we have it three times 
I have a strong affinity to a micro Korg on two counts: 1) I used to own an MS2000, my first synth and loved it; 2) I have a MicroMonsta 2, a Microfreak, and Microgranny so there is a theme there.
I’m sure it sounds great, but it is physically too large for my needs.
I’ve edited the top post with the full list of recommendations. Thank you for all of them.
I think the strongest contenders are:
• A second MicroMonsta 2 
::: but that might be boring to have the same synth twice.
• Fred’s Lab Buzzzy::: super compact, affordable.
• Circuit Tracks::: from a few videos I think the synth engine is higher quality than op-z, plus opens some intriguing options for a different portable setup of Tracks + MM2… plus has audio in and some fx.
• JX-08::: sounds lush. Has audio in.
With an outside chance on DSI Tetra or Virus Snow.
I’m also interested in model:cycles and blackbox for entirely different reasons.
Feel free to rubbish the above list based on sound quality and portability 
Let’s see what the new year brings…