Filters. VCAs. Envelopes. Line level inputs and outputs... especially if you're sampling externally.
The Batumi has an expander called the Poti. It gives you front panel access to features only natively available via jumpers on the back. It's worth the few dollars and rack space.
Speaking of rackspace. Yours is entirely full. As a first rack you probably want about 50% of your case empty. As your knowledge grows, you'll want to add more modules.
The 1010 Music Bitbox makes for a great sampler as you can record loops of your sounds. It syncs to clock which is extremely helpful.
I would think of all the non-sexy modules like attenuverters, VCAs, , envelope generators, sample & hold, logic, etc. They are what makes modular special. Get an understanding of what they do and you'll be in good shape.