If possible, a tweak to disable placing liquids within an already-source block would be immensely helpful for filling places with liquid (like soul sand elevators).
Something that I'm not entirely sure is possible client side, but would be very helpful, would be to, essentially, place fluids somewhat mid-air - if you right click onto a source, it would treat it as "solid" and instead place it "against" it. I'm not sure what is possible with client-side inputs, but one can dream.

AFAIK and remember, bucket uses a method that causes a ray trace on the server side, instead of passing a clicked block position from the client. This means that it is possible to prevent bucket uses, like placing inside an existing source, but it's not possible to fake the placement position (other than faking the player looking direction). And it's not possible to place fluids in mid-air, like it is with normal block placement, as the bucket only places the contained fluid when clicked against a block face.

Another issue is that I think this still suffers from server vs. client side player position or looking direction de-sync/delays, which caused or possibly still causes "bucket ghost uses" even in vanilla. So basically the "prevent fluid destroyal" tweak might still occasionally fail due to a slight position de-sync compared to the server's state of things.

Ah, I expected the second not to work but not for such an odd reason. Didn't expect it to be completely look-based,