We need a bigger inventory or stashtabs (like our treasure chest)

Very simple.
Less runs to town, no more overflowing inventories just by picking up currencies, more killing monsters, more money per stashtab for grinding gear games.

I would get rich if i get 1% money what people would pay for inventory stash tabs...

This is where Grim Dawn does it better than everybody else. Larger inventory space plus additional bags you can carry on your character so you can collect the gear and bring enough mats to swap your gear on the fly without having to identify it and go to your stash in order to upgrade it. It makes for more time actually playing the game and less idle time in town.