Conditional Drop Box

I am currently working on a spreadsheet and would like to make a cell that can auto-input a value (like an IF or VLOOKUP formula) but also has a conditional drop down list. Both of these would be based upon the same previous cell with its own drop down function. My main issue is that I do not need the drop down list for every condition selected.

For example: say the first cell reads "AAA". AAA would have an automatic input of "1". However, if "BBB" were selected I would need the box to create a drop down with options "2", "3", "4", and "5".

I was able to come close by utilizing an offset formula in the drop down source and an IF function in the cell. However, once a value was selected from the drop down menu it did not clear out to the original formula but rather replaced it with the value permanently. I would like for it to change with each selection while maintaining the original formula.

I realize I am probably explaining this terribly so I have attached a simple sample. Any help would be greatly appreciated!