Absolutely get a matrix mixer (for example the Doepfer A-138m). It won't seem sexy or all that necessary until you start messing with it and then it will open up worlds of possibilities.
Based on your last post, maybe a small rack just for mixing/sculpting the sounds produced by your other devices, and for creating/manipulating modulations that end up feeding your other stuff, would be worthwhile. Your rack doesn't necessarily need to produce any sounds on its own, if you have enough other gear to do that. For example, even if all your rack does is create and manipulate modulation that then feeds the other gear, it could provide a great way to interact and control things in a tactile way. Off the top of my head, maybe include the following:
- A joystick for creating and manipulating modulation directly (for example, the Intellijel one)
- A matrix mixer for mixing modulation sources (again, the Doepfer one is a great example)
- Attenuverter/offset for macro control (for example, Happy Nerding 3x MIA)
- A sophisticated envelope generator like the Xaoc Zadar
- Some LFOs (lots of great choices here)
- Probably some kind of function generator like Maths or Rampage
You could pack all that into a relatively small case and have loads of hands on (and hands off, if you choose) control over all your other gear. And then later if you feel like expanding your eurorack capabilities all of these will be essential anyway.