Well I think it cannot be Gate 23 area regardless

However, when I pass though HKIA last time, I found a stair and lift near Gate 27 (or 31, forgot) area which goes towards the Level 6, which probabaly the location for next longue.

Still I found it lack of reason for CX to open 4th airside Longue (Wing, Pier and G16), it only make sense if

1. Closing G16: How many of us go there anyway?
2. Taking over JAL Sakura (part of their cost cutting)

Which CX will able to better serve the airport with

Pier (Gate 1 - 19, 5xx)
** Cabin (Gate 20 - 50)
Wing (Gate 40 - 70)