That was my proposal. BPJ <> Larkin. Better to start from
BPJ.
BPJ ITH has no space already, needs to take care of its other
services 75, 176, 184, (700), 920, 922, 970, 972, 973, 975 and 979
first. None of these services are fit to be extended away from BPJ
(probably except 979 that could be extended to BB). The current bus
park may still be useful for some southbound trunks, but the
possibility isn't too high.
The other possibility is to let 170 start from BB and travel via
178's route, with 178 shortened to Boon Lay -- BB and a new feeder
replacing 178's Woodlands section. With the rest of UBT/BT/Dunearn
Rd covered by 67 and DTL2, transfers can be done at BPJ.
Or, why not extend 160 to Larkin and make it loop once around
BPJ? Then 170 can be made semi-express (or simply withdrawn,
following your idea)