Fish room!

Central filtration is convenient and fine if all your fish like the same water conditions, but I prefer the flexibility of separate filters so I can adjust hardness, pH, temp as needed for different species.

I use mostly air-lift type filters: foam or gravel in a plastic container with a pipe, all running from one air pump. (Get TWO air pumps - one for backup!) I use a few hang-on power filters for breeding gravel-spawning shiners and darters. I'd probably still be using my Dynaflows if the nylon ball bearings were still around!

Also you might want a refrigerator/freezer for live and frozen foods.