Ability to nominate MARC tags for "Local use", so existing data wouldn't get overwritten or deleted during data import. New data will get added as duplicate tag or subfield (depending on the setting preference for each "Local" MARC tag).
I am quite sure there already is a bug with some patches for a feature like that, but I don't manage to find it this morning.
I think bug 10477 might allow to do this, but not marking this duplicate as I am not completely sure.
*** This bug has been marked as a duplicate of bug 14957 ***