Every time I think about what could be missing from the case, I catch myself and realize that it's already there. I think you're going to find that you'll want either more modulation sources, a filter, or another mixer. I'd recommend looking into the latter as a replacement for the uStep or Branches - you won't always want to use Tides as a chord generator, but when you do, it'll allow you to squeeze all three voices out of your system instead of just one.
You may also want to look into a unity mix in 1u if it fits - in small systems, it'll let you change the base note and have a couple of sequences moving in the same key (if your sequencer doesn't already allow for that). It could also serve as a down and dirty mixer for multiple voices.
I hope this helps - I think, even if you bought this today as is, you'd have a wonderfully capable machine.