Kaoss Pad is nice- but playing in sequences with keys is better if you don’t already have that option. But, if you aren’t going to use it, in the long run (I never use my MIDI keys with the A4…) get a good FX pedal. The problem with the KP3 is that it’s more of a super Lo-Fi sampler that requires you to run audio through it for it to do much- most of the FX are useless if you aren’t running your whole set up through it. I’ve also had the problem where if it’s a send from and back into the OT (and not a mixer), it makes it so the OT is muted unless you’re triggering the effect on the pad. I mostly just use the delays, the looper, and the filter stuff for transitions- but you have enough to worry about with 4 Elektron boxes.
You need a more passive FX box that midi syncs and can be controlled from the direct CC controls of the Octatrack. I’d go with a Eventide Space Echo or some kind of reverb or distortion depending on what kind of music you make.