Got the USB/Thunderbolt ports on the new iMac sorted. One USB 3·0 is used to connect an unpowered USB 3·0 4-port hub that's clamped to the side, all ports face forward.
The other USB 3·0 port feeds the Logitech 15-watt 2.1 subwoofer stereo speaker/soundcard/powered USB 2·0 3-port hub thingie. One of its USB 2·0 ports is free, the others are used by the scanner & Apple DVD drive.
Nothing is currently connected to the front-facing USB 3·0 hub, they're for backup drives as needed & USB thumb drives. One of the two Thunderbolt 3 ports is used, connecting via T/Bolt 3-T/Bolt 2 adaptor then a T/Bolt 1 to FW 800 adaptor/FW800 cable to an external HD that's for storage.
Four USB 3·0 ports were not adequate. The SuperDrive & three ext HDs, one connected, the other waiting their turn all sit to the right of the old camera case that's used to elevate the foot of the iMac to bring it up to proper working height. The Fraudband* router/modem is on the wall behind the USB drives as is the AirPort Express which makes the USB laser printer into a networked one.
The cordless phone system plugged into the router/modem has its base station on the left, between the iMac stand and the scanner.