@Wahaha360 @fabio
TX is 20mm with 16mm core 13 tubes 22fpi (shroud to core difference is 2mm each side)
GTS is 30mm with 20mm core 14 tubes 16fpi (shroud to core is 5mm each side)
Hardware labs L series (bitspower leviathan) is exact same as GTS but 12 tubes.
EK classic is the same as L series but 27mm (so shroud is 3.5mm each side?)
Koolance CU720 is 30mm with 20mm core 12 tubes 30fpi (shroud is 5mm each side).
So lowest shroud height of 2mm each side is possible but most choose 5mm. And by the way, hardware labs is the OEM for all but the XSPC above. The XSPC performs very well despite thinner core, so if we are talking fully custom radiator for AIO solution, my dream would be:
17mm overall thickness, 15mm core, between 22-30FPI (say 25?), 12 tubes. And the key is shroud to core height of 2mm on one side and 0mm on the other. So give up push pull option but gain 2x A12x25 possiblity. But some might wonder if they wanted to put the fans on the other side (push instead of pull or vice versa), the solution is easy: 4 ports totals like the leviathan SF, so you simply flip the radiator around and still have ports on the other side. This would be END-GAME radiator.
Edit: adding alphacool ST25 (which I assume is same as st30 with shorter shrouds, I think safe to assume since ST30 has just a 16mm core in 30mm body) 25mm overall, 16mm core, 16fpi, 14 tubes, so shrouds of 4.5mm each side.
Edit 2:
@stanleyguan has a great point of the 17mm radiator on taobao, the core is flush with screw hole shrouds. Simply keep fins and tubes out of the area of the screw holes and you can have a 17mm core, high fin density, full size fan capable, ultimate SFF radiator.