Hmm.
Interesting, but...
Here is a thought. While some arpeggiators are great, I find them limiting unless you program them deeply by not only "hit notes" at specific time, but automate their sound / FX envelopes. It would require quite a bit of programming to make it sound right. There are some powerful third party plugins that you can make perform with BIAB arrangement (within BIAB).

Here is an alternative idea, which in my opinion has to come first. It is somewhat within the line of pattern / arpeggio world. There are great hundreds (or thousands?) of BIAB MIDI styles. It is possible, but very awkward of extracting individual MIDI stems out of these styles. If it was a straight forward "easy" process, users would have access to a huge library of editable midi patterns that can be selectable within the mixer and assigned to any instrument.

This way the workflow could be similar to picking RTs. Ideally, it can work like this: You would preview and assign midi line to a synth of your choice right from the picker and when you click open (or generate) from the picker, it will place that midi line on a separate track, including the synth you chose in the picker. And of course it should follow all the chord changes and "Settings at the bar"

Main benefit of this approach is that enormous library of patterns (lines within Midi styles) already exist in BIAB and they are more will feel more "natural" than pattern maker, because many of these were played by humans. Also being a MIDI you can edit it with precision.

P.S. If this is done right, a MIDI pack(s) can be made specifically for arpeggios for some modern stuff. Similar to loop packs, but far more flexible.