Sure...you just need to have common signatures with the data.
So, in this case that you stated, your table in the else part of your If statement will have a signature of "Value", so you need to shape the true part of the If to equal that...like so:
If(varXYZ, RenameColumns( ShowColumns(SharepointList, "ColumnName"), "Value"), [“different”, “data”, “shape”] )
In this case, the ShowColumn will return a table of "ColumnName", we then rename it to "Value" as that is the result of the manual table.
You could do it the other way around as well...put the rename columns around the manual table and rename the "Value" column to "ColumnName".
I hope this is helpful for you.