Well first off you need to cure this …
You were talking about the new BQ-10 here, but it is the same x 10 with what you are asking about.
Other stuff – read the posts upthread here ( minus the off-topic E’naut weeds ). Stuff like adding a second fixed filter bank, etc.
Which begs the question, do you want to do stereo ?
Are you looking to be a purist, with a System 55 copy, or are you open to improvement and variety ? I will assume the later for the rest of this post.
If you are using the sequencer, you should consider CV note quantizers as i posted about over here ( i know you were already there and read that, but for completeness ). Behringer doesn’t sell them, so you’d have to buy other brands. There are a ton of alternatives. You can consider the simple sort in my post, or more elaborate things like as for instance, and i throw this out only to give you ideas, the Instruo Harmonaig ( yes it is expensive ) which is chordal, so you can do chord progressions. To many varieties of options with that to list here.
Consider MIDI interface options, if you have sources of external MIDI. ( Actually the new Arturia Keystep Mk2 ( thread ) with all the new options, would be tremendous bang for buck, with both MIDI and CV out – as alternative to using the 960 sequencer all the time. Plus you get quantization from MIDI to CV, in that approach. )
Also check out this post, for alternate Behringer voice options that might be fun to add to the traditional System 55 voices. Also look at the various other Behringer modules listed there, and elsewhere in that thread.
Then just play it by ear ( literally and figuratively ) and fill out your System 55 as you go.