The ability to replace text, tiles, or entities massively

You should be able to replace text in a script with different text, not unlike Notepad++'s search and replace feature. You should be able to replace tiles as well, e.g. replace all instances of tile 69 in this room or in the entire level with instances of tile 1337. You could replace entities the same way, i.e. replace all instances of entity 16 with entity 17. You should be able to replace entities and tiles in a selection selected by the selection tool, too.