to a large extent any other sequencer will complement the BS... I have Marbles, step fader, BSP etc etc as well as a BS
things that are really helpful in this are precision adders (maybe a multi-channel one) so that you can transpose sequences (can't remember if you can do this straight into the BS)
a second or subsequent sequencer can be used as a sequenced modulation source as well as more sequenced pitch (if/when you have enough voices)
a cv addressable quantizer can also be used to alter sequences - again multi-channel is useful here
I use a sinfonion as a chord pattern sequencer and multi-channel quantizer (and arpeggiator) and I use a second sequencer (the BSP) to change chord patterns - which I clock from the sinfonion (a gate out on the last bar of each pattern) which means it advances very slowly and can be used to change other things - via a switch for example
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities