Thanks to TLC for asking us. I would be interested in what are the good sellers now. It would be great if they could use old moulds for PAB parts and have non-production pieces available only on PAB, which would make PAB a real alternative to bricklink. I guess PAB is a warehouse full of small-lot production runs, so they could experiment with batch production of a few high-demand old pieces like classic space parts and see how they sell. I guess this is a commercial decision ultimately - my observation is that the really high brick-count MOCs tend to be castle or space themed, so commercially I would look to please the builders who would buy in bulk and give the big revenue sources.
for me, I would love to see-
Fleshie heads and hands
more roof slopes in different colours. 2x2x3 inside corner bricks -I wish
more doors and windows/cockpits - please trans yellow cockpits!
some specialist pieces from current ranges - barbed wire, laptop, telescope
more arches
more plants
For colours, more brown, silver, old grey, sand green, dark green, dark blue
If there is some physical limit on the number of stock items they carry, I would recommend to cut back on primary colour bricks and plates as they are quite easily available.