If you're using your Eurorack as more of an expander for your existing synths, I'd concentrate in two areas: modulation and effects.
Your effects do not have to be Eurorack but if they have CV modulations in them, they will be much more useful. There is a TON of effects modules out there. Z-DSP, Magneto, Clouds (and clones), ErbeVerb, Rainmaker are some of the popular models thrown around. Also consider overdrive/distortion modules as effects. A little dirt doesn't hurt.
As far as modulation, the sky is the limit on that as well: extra LFOs, extra envelope generators, mutes, sequencers (they are great for more than just controlling pitch and triggering envelopes, CV processors like the Worng Vector Space, etc.
You can also place additional filter types in your rack if you want to run your Grandmother synth through something different than the stock filter.
If you haven't bought any or all of the Mutable instrument stuff, I'd seriously recommend buying the much more compact clone versions of each module. They are generally cheaper and take up much less space.
Take a deep breath and start slow. You already have a decent collection of modules so see how you can augment what you have with them and what areas you feel they are falling short. It's hard to say "what you need" as everyone's set-up is as individual as the individual.