Dynamic Data Validation Source

PLAWMD 

This requires Excel 365 in order to leverage the new "Dynamic Array" formulas.  

I imported the team list in Screenshot 1 from the web.  Column 1 of that imported table is the starting point for the validation list.  

The formula for the data validation list is:

=SORT(FILTER(Standings___Detailed_View[NFL Team.1],NOT(ISNUMBER(MATCH(Standings___Detailed_View[NFL Team.1],tbl_UserInput[Team],0)))),1,1)

That formula references the table on the second "Input Form" tab called tbl_UserInput (shown below).  Once the team name is copied to that table, it will no longer be visible in the validation list:

For the purpose of keeping this simple, the validation list is referencing against team names in a single table.  If the intent is to have 1 data validation cell for each week's schedule, the dynamic validation list formula posted above will require either more complexity or need to be duplicated once per weekly tab.  Here is the portion of the formula that would need to be revised to point to different tabs/tables.  Specifically, tbl_UserInput[Team] would need to be revised depending on the tab.   

NOT(ISNUMBER(MATCH(Standings___Detailed_View[NFL Team.1],tbl_UserInput[Team],0))))

Lastly, the syntax for the source of the dropdown list follows.  Note the "#" at the end of the cell reference means it is referencing the entire dynamic array, as opposed to cell X2.  I also disabled the "show alert..." on the Error Alert tab.  

Hope this helps, or at least gets you started.  

Dexter