That would however somewhat run afoul of the way teh vault is set up, third slot always requires double the effort of the second…
2/4/8 raidbosses
1/4/8 Dungeons (which IMO should also be 2/4/8)
But then for delves linear 3/6/9 unlock? Also, the third delve slot would then be an outlier for the additional reason that it´s now requiring only 1 more complete while the second already requires two more. 