Just to confirm a few ideas that have already been mentioned.
If you want something that has a lot of possibilities, you should definitely get Ornament and Crime. It does so many things from sequencing, LFOs, and much more. You can change the firmware to make it (using Teensy) into a drum/rhythm module even.
Secondly, as @Ronin1973 mentions, a VCA could be useful in many situations. I have a Intellijel Quad VCA and indeed it gets used a lot. However, I noticed that Doepfer have what looks to be an even more (useful) VCA with the A-135-2. Although I don't have this (yet) it looks as though it has a more interesting set up. It has attenuators, like the Quad VCA, but more interesting two extra mix outputs, 'Selected' (takes the signals from Inputs, provided they've not been diverted via an Output) and 'All' (all signals going to the Inputs). This can/could be particularly useful, and the Quad VCA doesn't have this possibility.
It's also a lot cheaper too.