The Octatrack is a beast of a central music making piece.
I’ll tell you about my setup, which I think is reasonably full-featured (minus loop layering and other OT goodies) and maybe you’ll get some ideas.
Digitakt - Sampling/Drum centerpiece (probably redundant if you’re rocking the OT)
Digitone - Sometimes a multi-timbral FM synth, sometimes holding down the fort with drums, bass, core song stuff, depending. This is a unique little beast and a favorite piece of kit for me.
MicroMonsta 2 - 2x6 or 1x12 voice VA/FM/WT synth. A little light on the FM or WT, but not really critical since I mostly use it for poly (virtual) analog sounds. Sometimes sequenced from the Digitone, sometimes played live, and on rare occasions, I’ll sample something from it into the Digitakt. To me, this little wonder sounds so good, I find myself getting lost just playing it sometimes.
CIrcuit Mono Station - For when I just HAVE to have some real analog. Single voice or paraphonic. Plus, with its parameter locking, patch flipping sequencer, I can get CRAZY drum/percussive/tonal or mixes of all three in sequences that have an almost modular sequenced analog sound. GREAT for sampling from into my Digitakt. I could imagine some VERY interesting stuff if you sampled entire sequences (melodic, bass, drones, drums, percussion, crazy noises) from something like this into an Octactrack as loops, then went to town with madness piled on madness.
I also have a couple of keyboard controller choices, a Keystep for portability and a full sized Novation for more “serious” playing.
I don’t have some things in my current core setup that others might deem critical, like a rompler, a strong WT synth, time-stretched loop layering, etc., but for the sounds and music that I’m creating right now, my small focused setup is keeping me creative without going down a gear rabbit hole.
I don’t know how much of this is relevant to your Octatrack-based needs, but maybe you’ll get some ideas from what I’m up to with my particular choice of modules, ignoring the Digitakt, which I’m guessing you don’t need.