I wasn't able to guess what you meant by average auto increase or decrease, but if you are using a recent version of Numbers then you can use the new SEQUENCE function to automatically generate a sequence of dates or number of days, something like this:

In cell A2:
=SEQUENCE(DATEDIF('Start-End'::$B$1,'Start-End'::$B$2+1,"D"),1,'Start-End'::$B$1,1)
In cell B2:
=SEQUENCE(DATEDIF('Start-End'::$B$1,'Start-End'::$B$2+1,"D"),1,1,1)
The SEQUENCE function takes these parameters:
SEQUENCE(number-of-rows,number-of-columns,start,step)
The number-of-rows here can be derived using DATEDIF as above. An alternative is to subract the start date from the end date and use DUR2DAYS, like this:
=SEQUENCE(DUR2DAYS('Start-End'::$B$2−'Start-End'::$B$1)+1,1,'Start-End'::$B$1,1)
Note that I have a +1 in the formulas because I assumed you wanted to include the end date.
Numbers will warn you with an error triangle if you don't have enough rows to hold the sequence of dates between the start and end dates that you enter.
More on the functions here:
SEQUENCE - Apple Support
DUR2DAYS - Apple Support
SG