As an owner of the Mavis and a big fan, I would still highly suggest keeping that in its little standalone case it came with and filling out that area with more practical stuff like what was mentioned above. You should definitely try using it as an effect as well - not only for the filter, wavefolder, and built in modulation, but also for the sample and hold, which makes a great sample rate reduction effect if you put audio in and sample it at audio rate, kind of like a bitcrushed sound. It's a great synth, but especially when it comes with its own decksaver and all that, you might as well just sit it on the side like I do with my Mavis and Pico System III. Save the rack for things that can't go anywhere else - almost no one makes, say, standalone VCAs or attenuverters that are fun and easy to use (there are some "0hp" Eurorack tools you can find, but they're generally small and awkward). The exception is matrix mixers, which are an amazing modular utility and can absolutely be purchased standalone. Even then, though, you'd probably benefit more with that and some utilities in the rack and Mavis on the side than the other way around.