formulize | : waning_crescent_moon : formula ui generator

My decision variable, which is to be minimized is this:

I want to do the following: I have a table = Listobject which has a column called MasterID. Some columns have the same MasterID and some have even non. I need to manually add the missing MasterIDs.

I want take a row where the MasterID is empty and then I want to click on the column MasterID and select an ID for this column. It can either be an existing ID. Which is the unique lsit of used MasterIDs in the hole listcolumn OR it can be a new MasterID. If a new ID is selected it should be the next integer from the biggest MasterID. So if the highest masterID up until now was 1000 then the new one should be 1001.

So I wanted to know if there is a way to use data validation in order to suggest me the next bigger MasterID or all existing. Since the already filled MasterIDs are randomly distributed I need to make this into a single formula.

Lets formulize this a little:

IF the cell is NOT EMPTY it can be whatever it wants to be, ELSE the cell needs to be one of the values used in the listcolumn OR the MAXIMUM of the Listcolumn +1.

If possible I would like to use a dropdown list.

I have tried this with a data validation list option but I couldn't figure out how. I know there needs to be a structur like this: