When renaming multiple files in Dolphin, you have the option to input "#" for incremental number. I think it will be a really big improvement if we had another symbol, for example "%" (or anything else that can't or shouldn't normally be used in a file name due to compatibility concerns) - and that would leave the original file name at that spot.

For example, if we have the files:
"icon_16.png" ->                                   -> "icon_16_old.png"
"icon_32.png" -> rename to "%_old" -> "icon_32_old.png"
"icon_ 64.png"                                       -> "icon_64_old.png"

This will be documented below with the text "% will be replaced by each file's original name."

Additionally, if we provide an extension in the rename, it should replace the file's current extension:

"icon_16.png" ->                                   -> "icon_16.old"
"icon_32.png" -> rename to "%.old" -> "icon_32.old"
"icon_ 64.png"                                       -> "icon_64.old"

Currently if you rename "icon.png" files to "file#.jpeg", it results in "file1.jpeg.png", which is probably not what the user intended.

There are probably more possible  improvements to batch renaming, but these are the most fundamental off the top of my head, and the ones that would be the most useful to the most people.