Enhance the Append Tool's functionality by allowing users to utilize Python or Arcade expressions to fill in output fields that do not have a 1:1 match between the two data sets.
Example:
Many exploratory data analysis feature classes start from source data with their own attribution. In the process of analysis, an individual may add/remove some fields but otherwise keep the data similar. One workflow is to then append additional records from the source data, selecting "use the field map to reconcile field differences" is useful to just add the new data, but then the user has to go into the table, highlight the newly added records and then run field calculations for each of the new columns their analysis dataset might have.
Or a different example, you have two datasets that are similar, but one is missing some relevant individualized fields-- --or has the same data in incompatible fields (it has a string and you need a numeric). You can create these fields easily enough with a field calculation expression and should be able to do this at the point of entry within the append tool.
The enhancement would look this, for a field(s) you could write in expressions to fill out the data programmatically as it uploads to your established dataset.