FCAL, VTX, TPC - the beam tube thickness and material
SIT and forward disks - in particular having the first disks closer to the VTX would be better
and at the same time the SIT could be slightly shortened.
TPC - cables, support, positive ion tolerance etc.
HCAL, optimization - tail catcher
LCAL - takes a lot of space without active material others ? - electronics of LCAL
others